Ports: Upgrade LLVM to version 13.0.0

With this update, we now use our custom `serenity` Clang target, which
means that all system-specific compilation options (e.g. default PIE,
header search paths) will be handled automatically.

This port has been tested to build `Source/little` on all 4
toolchain-architecture pairs. Furthermore, `lib(std)c++` headers are
picked up correctly and our AK headers can be included without any
issues.

Due to recent kernel fixes related to memory-mapped files, the LLD
linker can now be used by default, so there's no need to also build the
GCC port alongside this.

Although our patches cover building libLLVM as a shared library, this is
currently not enabled by default, as DynamicLoader is very slow in
dealing with such a large number of relocations.

Ports/llvm/patches/ReadMe.md

@@ -1,9 +1,54 @@
 # Patches for LLVM on SerenityOS
 
-## `remove-wstring.patch`
+## `build-crt.patch`
 
-Removes `wstring`s from the source code, as SerenityOS doesn't support them yet.
+This patch lets us use LLVM's `crtbegin.o`/`crtend.o` implementation.
+
+### Status
+- [ ] Local?
+- [x] Should be merged to upstream?
+- [ ] Resolves issue(s) with our side of things
+- [ ] Hack
 
 ## `insert-ifdef-serenity.patch`
 
 This patch adds several defines in order to omit things not supported by SerenityOS.
+
+### Status
+- [ ] Local?
+- [ ] Should be merged to upstream?
+- [X] Resolves issue(s) with our side of things
+- [x] Hack
+
+## `remove-version-script.patch`
+
+Instructs the linker to not build LLVM shared libraries (`libclang.so`, `libLTO.so`, etc.) with
+symbol versioning, which our dynamic linker does not support.
+
+### Status
+- [ ] Local?
+- [x] Should be merged to upstream?
+- [X] Resolves issue(s) with our side of things
+- [ ] Hack
+
+## `remove-wstring.patch`
+
+Removes `std::wstring`s from the source code, as our libstdc++ does not support it.
+
+### Status
+- [ ] Local?
+- [ ] Should be merged to upstream?
+- [X] Resolves issue(s) with our side of things
+- [X] Hack
+
+## `toolchain.patch`
+
+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 building position-independent executables by default.
+
+### Status
+- [ ] Local?
+- [x] Should be merged to upstream?
+- [ ] Resolves issue(s) with our side of things
+- [ ] Hack