Meta: Switch to a SuperBuild that splits host and target builds

Replace the old logic where we would start with a host build, and swap
all the CMake compiler and target variables underneath it to trick
CMake into building for Serenity after we configured and built the Lagom
code generators.

The SuperBuild creates two ExternalProjects, one for Lagom and one for
Serenity. The Serenity project depends on the install stage for the
Lagom build. The SuperBuild also generates a CMakeToolchain file for the
Serenity build to use that replaces the old toolchain file that was only
used for Ports.

To ensure that code generators are rebuilt when core libraries such as
AK and LibCore are modified, developers will need to direct their manual
`ninja` invocations to the SuperBuild's binary directory instead of the
Serenity binary directory.

This commit includes warning coalescing and option style cleanup for the
affected CMakeLists in the Kernel, top level, and runtime support
libraries. A large part of the cleanup is replacing USE_CLANG_TOOLCHAIN
with the proper CMAKE_CXX_COMPILER_ID variable, which will no longer be
confused by a host clang compiler.

.github/workflows/cmake.yml

@@ -118,14 +118,13 @@ jobs:
           ccache -s
       - name: Create build directory
         run: |
-          mkdir -p ${{ github.workspace }}/Build
-          mkdir -p ${{ github.workspace }}/Build/UCD
-          mkdir -p ${{ github.workspace }}/Build/CLDR
+          mkdir -p ${{ github.workspace }}/Build/${{ matrix.arch }}/UCD
+          mkdir -p ${{ github.workspace }}/Build/${{ matrix.arch }}/CLDR
       - name: UnicodeData cache
         # TODO: Change the version to the released version when https://github.com/actions/cache/pull/489 (or 571) is merged.
         uses: actions/cache@03e00da99d75a2204924908e1cca7902cafce66b
         with:
-          path: ${{ github.workspace }}/Build/UCD
+          path: ${{ github.workspace }}/Build/${{ matrix.arch }}/UCD
           key: UnicodeData-${{ hashFiles('Meta/CMake/unicode_data.cmake') }}
       - name: UnicodeLocale Cache
         # TODO: Change the version to the released version when https://github.com/actions/cache/pull/489 (or 571) is merged.
@@ -134,22 +133,38 @@ jobs:
           path: ${{ github.workspace }}/Build/CLDR
           key: UnicodeLocale-${{ hashFiles('Meta/CMake/unicode_data.cmake') }}
       - name: Create build environment with extra debug options
-        working-directory: ${{ github.workspace }}/Build
         # Build the entire project with all available debug options turned on, to prevent code rot.
-        # However, it is unweildy and slow to run tests with them enabled, so we will build twice.
-        run: cmake .. -GNinja -DSERENITY_ARCH=${{ matrix.arch }} -DBUILD_LAGOM=ON -DENABLE_ALL_DEBUG_FACILITIES=ON -DENABLE_PCI_IDS_DOWNLOAD=OFF -DENABLE_USB_IDS_DOWNLOAD=OFF -DCMAKE_C_COMPILER=gcc-10 -DCMAKE_CXX_COMPILER=g++-10
+        # However, it is unwieldy and slow to run tests with them enabled, so we will build twice.
+        run: |
+          cmake -S Meta/CMake/Superbuild -B Build/superbuild -GNinja \
+            -DSERENITY_ARCH=${{ matrix.arch }} \
+            -DSERENITY_TOOLCHAIN=GNU \
+            -DBUILD_LAGOM=ON \
+            -DCMAKE_C_COMPILER=gcc-10 \
+            -DCMAKE_CXX_COMPILER=g++-10 \
+            -DENABLE_ALL_DEBUG_FACILITIES=ON \
+            -DENABLE_PCI_IDS_DOWNLOAD=OFF \
+            -DENABLE_USB_IDS_DOWNLOAD=OFF
         if: ${{ matrix.debug-options == 'ALL_DEBUG' }}
       - name: Create build environment
-        working-directory: ${{ github.workspace }}/Build
+        working-directory: ${{ github.workspace }}
         # Note that we do not set BUILD_LAGOM for the normal debug build
         # We build and run the Lagom tests in a separate job, and sanitizer builds take a good while longer than non-sanitized.
-        run: cmake .. -GNinja -DSERENITY_ARCH=${{ matrix.arch }} -DENABLE_UNDEFINED_SANITIZER=ON -DENABLE_PCI_IDS_DOWNLOAD=OFF -DENABLE_USB_IDS_DOWNLOAD=OFF -DCMAKE_C_COMPILER=gcc-10 -DCMAKE_CXX_COMPILER=g++-10
+        run:  |
+          cmake -S Meta/CMake/Superbuild -B Build/superbuild -GNinja \
+            -DSERENITY_ARCH=${{ matrix.arch }} \
+            -DSERENITY_TOOLCHAIN=GNU \
+            -DCMAKE_C_COMPILER=gcc-10 \
+            -DCMAKE_CXX_COMPILER=g++-10 \
+            -DENABLE_UNDEFINED_SANITIZER=ON \
+            -DENABLE_PCI_IDS_DOWNLOAD=OFF \
+            -DENABLE_USB_IDS_DOWNLOAD=OFF
         if: ${{ matrix.debug-options == 'NORMAL_DEBUG' }}
 
       # === ACTUALLY BUILD ===
 
       - name: Build Serenity and Tests
-        working-directory: ${{ github.workspace }}/Build
+        working-directory: ${{ github.workspace }}/Build/superbuild
         run: cmake --build .
       - name: Show ccache stats after build
         run: ccache -s
@@ -159,12 +174,12 @@ jobs:
 
       - name: Create Serenity Rootfs
         if: ${{ matrix.debug-options == 'NORMAL_DEBUG'}}
-        working-directory: ${{ github.workspace }}/Build
+        working-directory: ${{ github.workspace }}/Build/${{ matrix.arch }}
         run: ninja install && ninja image
 
       - name: Run On-Target Tests
         if: ${{ matrix.debug-options == 'NORMAL_DEBUG'}}
-        working-directory: ${{ github.workspace }}/Build
+        working-directory: ${{ github.workspace }}/Build/${{ matrix.arch }}
         env:
           SERENITY_QEMU_CPU: "max,vmx=off"
           SERENITY_KERNEL_CMDLINE: "boot_mode=self-test"
@@ -189,5 +204,5 @@ jobs:
       - name: Print Target Logs
         # Extremely useful if Serenity hangs trying to run one of the tests
         if: ${{ !cancelled() && matrix.debug-options == 'NORMAL_DEBUG'}}
-        working-directory: ${{ github.workspace }}/Build
+        working-directory: ${{ github.workspace }}/Build/${{ matrix.arch }}
         run: '[ ! -e debug.log ] || cat debug.log'