[llvm] [llvm][docs] Correct the list of the available -fveclib= options to match with the reality (PR #168205)

[Aaron Ballman via llvm-commits]

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

LGTM, agreed with changing the driver to accept varying cases though.

https://github.com/llvm/llvm-project/pull/168205