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

[Paul Walker via cfe-commits]

paulwalker-arm wrote:

Yep, that looks broken as well.  I'll update the PR. I've done a grep for `OPT_fveclib` and I don't see any other uses so hopefully this is the last one. 

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