[LTO] Fix Veclib flags correctly pass to LTO flags (#78749)

Flags `-fveclib=name` were not passed to LTO flags.
This pass fixes that by converting the `-fveclib` flags to their
relevant names for opt's `-vector-lib=name` flags.

For example:
`-fveclib=SLEEF` would become `-vector-library=sleefgnuabi` and passed
through the `-plugin-opt` flag.