[clang] [clang][LoongArch] Add FreeBSD targets (PR #119191)

[WÁNG Xuěruì via cfe-commits]

xen0n wrote:

> > Also a `[Clang]` tag could be prepended to the PR title to make it clearer what part gets changed.
> 
> Not only clang, all of FreeBSD on LoongArch

It's not about *what can get built after the change*, but *what part of LLVM project this change applies to*, which is Clang. Look at the diff -- all touched files reside in `clang/` directory, that's it.

https://github.com/llvm/llvm-project/pull/119191