[clang] [RISCV][clang] Use fcf-protection flag in Multilib Selection (PR #205202)

[Kito Cheng via cfe-commits]

https://github.com/kito-cheng approved this pull request.

The PR itself is fine, but I noticed that clang and gcc handle `-fcf-protection=` in slightly different ways[1]. Clang treats it like a regular option: last-wins, but gcc processes it sequentially. For example:

`march=rv64gc_zicfiss_zicfilp -fcf-protection=branch -fcf-protection=return`

In gcc, it's equivalent to `-fcf-protection=full`, but in clang it's `-fcf-protection=return`.

However I think this should not become blocker for this PR, we may adjust after than once clang align the behavior with gcc for `-fcf-protection=`

[1] https://godbolt.org/z/TqY9hYo8d

https://github.com/llvm/llvm-project/pull/205202