[clang] [Clang][Driver] Fix target parsing for -fveclib=libmvec option. (PR #138288)

[Benjamin Maxwell via cfe-commits]

https://github.com/MacDue approved this pull request.

LGTM, thanks :+1: 

There's one other use of "LIBMVEC" in flang that looks suspicious, but I've not check if it's actually an issue:

https://github.com/llvm/llvm-project/blob/d6dbe7799e638702309e23fc7b73d4be2231e2ac/flang/lib/Frontend/CompilerInvocation.cpp#L194-L198

https://github.com/llvm/llvm-project/pull/138288