gn build: Add check-builtins target.

[pcc]

Tested on aarch64 Linux and x86_64 Linux.

[aeubanks]

wow this is very messy, feels like there should be a better way to do this on the CMake side

[pcc]

wow this is very messy, feels like there should be a better way to do this on the CMake side

Yeah, or maybe even in the tests themselves, maybe they should be using weak symbols for the builtins and return unsupported if the symbol is undefined at runtime.

[nico]

This breaks llvm/utils/gn/build/sync_source_lists_from_cmake.py output, it still prints output like:

[gn build] Port 6d03f51f0c59 -- https://github.com/llvm/llvm-project/commit/6d03f51f0c59
  llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn
   add:
    "extendhfdf2.c",

[pcc]

Isn't that a new file? I would expect it to try to add it to the _unused set.