[clang] [Toolchains][FreeBSD] Honor system libgcc (PR #181972)

[via cfe-commits]

aokblast wrote:

> It would have been better if we had always used clang's own naming and place conventions for its builtin libraries, but at the time we were pivoting from gcc to clang, we had to go for the least intrusive method, which was to keep the name `libgcc.a`, but put `libcompiler_rt.a` (aka `libclang_rt.builtins.a`) in its place. (I believe it was even a MK_xxx toggle at some point).
> 
> That said, I'm also wondering what this patch is supposed to achieve: is it purely to fix `--print-libgcc-file-name`? Or also to support customizing where `libgcc.a` is found?

It currently supports both cases. If people have their builtins.a, it can be detected. Also, if people try to rename libclang_rt.builtins.a -> libgcc.a. It can also be detected. But in my case, I just want to solve --print-libgcc-file-name.

https://github.com/llvm/llvm-project/pull/181972