Ports: Add zig port :^)

:yakkie:

The build process for the Zig compiler is more involved than most of
the other ports, because the Zig compiler is mostly self-hosting. In
order to build it, the zig-bootstrap build system is used, which does
the following:

1) Build LLVM for the host OS;
2) Build Zig for the host OS with the SerenityOS target enabled;
3) Build zlib, zstd and LLVM for SerenityOS using `zig cc` as the C/C++
   compiler;
4) Build Zig for SerenityOS using the host Zig.

A few hacks are required in order to tell `zig cc` and zig about what
Serenity's libc looks like in the build process, but other than that
it's fairly straightforward. All of the patches that are included with
this commit are Zig-upstream ready once the LLVM patches are upstreamed.

Ports/zig/patches/ReadMe.md

@@ -0,0 +1,109 @@
+# Patches for zig on SerenityOS
+
+## `0001-Add-support-for-building-LLVM-on-SerenityOS.patch`
+
+Add support for building LLVM on SerenityOS
+
+Adds SerenityOS `#ifdef`s for platform-specific code.
+
+We stub out wait4, as SerenityOS doesn't support querying a child
+process's resource usage information.
+
+## `0002-Add-triple-for-SerenityOS.patch`
+
+Add triple for SerenityOS
+
+
+## `0003-Add-support-for-SerenityOS.patch`
+
+Add support for SerenityOS
+
+Adds support for the `$arch-pc-serenity` target to the Clang front end.
+This makes the compiler look for libraries and headers in the right
+places, and enables some security mitigations like stack-smashing
+protection and position-independent code by default.
+
+## `0004-Default-to-ftls-model-initial-exec-on-SerenityOS.patch`
+
+Default to -ftls-model=initial-exec on SerenityOS
+
+This is a hack to make Clang use the initial-exec TLS model instead of
+the default local-exec when building code for Serenity.
+
+This patch should be removed when we implement proper TLS support.
+
+## `0005-Add-support-for-SerenityOS.patch`
+
+Add support for SerenityOS
+
+This commit teaches libc++ about what features are available in our
+LibC, namely:
+* We do not have locale support, so no-op shims should be used in place
+  of the C locale API.
+* The number of errno constants defined by us is given by the value of
+  the `ELAST` macro.
+* Multithreading is implemented though the pthread library.
+* Use libc++'s builtin character type table instead of the one provided
+  by LibC as there's a lot of extra porting work to convince the rest of
+  locale.cpp to use our character type table properly.
+
+This commit is an adaptation of the LLVM patch by Daniel Bertalan to fit
+the layout of the zig-bootstrap project.
+
+
+## `0006-Allow-undefined-symbols-on-SerenityOS.patch`
+
+Allow undefined symbols on SerenityOS
+
+Allow undefined symbols in LLVM libraries, which is needed because only
+stubs are available for SerenityOS libraries when libc++ and libunwind
+are built.
+
+## `0007-Support-building-shared-libLLVM-and-libClang-for-Ser.patch`
+
+Support building shared libLLVM and libClang for SerenityOS
+
+This patch tells CMake that the --whole-archive linker option should be
+used for specifying the archives whose members will constitute these
+shared libraries.
+
+Symbol versioning is disabled, as the SerenityOS loader doesn't support
+it, and the ELF sections that store version data would just waste space.
+
+## `0008-Add-SerenityOS-to-config.guess.patch`
+
+Add SerenityOS to config.guess
+
+
+## `0009-Prevent-the-use-of-POSIX-shm-on-SerenityOS.patch`
+
+Prevent the use of POSIX shm on SerenityOS
+
+POSIX shm is not supported by SerenityOS yet, so this causes a
+compilation error.
+
+## `0010-llvm-Implement-bigint-to-LLVM-int-for-32-bit-compile.patch`
+
+llvm: Implement bigint-to-LLVM int for 32-bit compiler builds
+
+The conversion to DoubleLimb is necessary due to LLVM only accepting
+64-bit limbs for big integers. Since we need some space to store it, we
+also have to allocate. This is an unfortunate penalty that 32-bit
+compiler builds have to take.
+
+## `0011-Add-SerenityOS-target.patch`
+
+Add SerenityOS target
+
+Named "serenity" within the code to match what LLVM says.
+
+## `0012-Implement-SerenityOS-support-in-std.patch`
+
+Implement SerenityOS support in std
+
+
+## `0013-build-Adjust-build-process-for-SerenityOS.patch`
+
+build: Adjust build process for SerenityOS
+
+