Add GCC backend using libgccjit

[BreadTom]

GCC has a JIT C API which allows Zig to compile to GCC using gcc_jit_context_compile_to_file().

Once Zig no longer depends on LLVM, this can be merged.

• make the main zig executable no longer depend on LLVM, LLD, and Clang libraries #16270

I do not know how to make zig link with libgccjit.

• Link Zig with libgccjit
• Convert most Air.Inst.Ref to gcc_jit_type
• Convert most Air.Inst.Tag to expressions
• Generate function like llvm.FuncGen

[alexrp]

Setting aside that this is a non-starter due to the GPL, this would just be replacing one set of problems with another. The end goal is for Zig to not link to any third-party compiler infrastructure library.

[BreadTom]

Setting aside that this is a non-starter due to the GPL, this would just be replacing one set of problems with another. The end goal is for Zig to not link to any third-party compiler infrastructure library.

Forgot about the GPL licensing in the header. Going to close this pull request.

[andrewrk]

If GCC has a stable IR, equivalent to LLVM .bc files, then a gcc backend can be considered