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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Adding std::tolower on each relevant string comparisons in the C++ files would look uncomfortable enough,

We have StringRef::equals_insensitive 

> but adding lowercase counterparts to each available veclib option in the Options.td file would look even more uncomfortable (and concerning to the most of the reviewers). 

I don't see how this is burdensome 

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