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

The command line reality is this:

$ clang -c prog.c -fveclib=accelerate
error: invalid value 'accelerate' in '-fveclib=accelerate'

$ clang -c prog.c -fveclib=Accelerate
prog.c:1:2: warning: This is only a test [-W#warnings]
    1 | #warning This is only a test
      |  ^
1 warning generated.

$ clang -c prog.c -fveclib=libmvec
prog.c:1:2: warning: This is only a test [-W#warnings]
    1 | #warning This is only a test
      |  ^
1 warning generated.

$ clang -c prog.c -fveclib=LIBMVEC
error: invalid value 'LIBMVEC' in '-fveclib=LIBMVEC'

$ clang -c prog.c -fveclib=massv
error: invalid value 'massv' in '-fveclib=massv'

$ clang -c prog.c -fveclib=MASSV
prog.c:1:2: warning: This is only a test [-W#warnings]
    1 | #warning This is only a test
      |  ^
1 warning generated.

$ clang -c prog.c -fveclib=sleef
error: invalid value 'sleef' in '-fveclib=sleef'

$ clang -c prog.c -fveclib=sleefgnuabi
error: invalid value 'sleefgnuabi' in '-fveclib=sleefgnuabi'

$ clang -c prog.c -fveclib=SLEEF
prog.c:1:2: warning: This is only a test [-W#warnings]
    1 | #warning This is only a test
      |  ^
1 warning generated.

$ clang -c prog.c -fveclib=darwin_libsystem_m
error: invalid value 'darwin' in '-fveclib=darwin_libsystem_m'

$ clang -c prog.c -fveclib=Darwin_libsystem_m
prog.c:1:2: warning: This is only a test [-W#warnings]
    1 | #warning This is only a test
      |  ^
1 warning generated.

$ clang -c prog.c -fveclib=armpl
error: invalid value 'armpl' in '-fveclib=armpl'

$ clang -c prog.c -fveclib=ARMPL
error: invalid value 'ARMPL' in '-fveclib=ARMPL'

$ clang -c prog.c -fveclib=ArmPL
prog.c:1:2: warning: This is only a test [-W#warnings]
    1 | #warning This is only a test
      |  ^
1 warning generated.

$ clang -c prog.c -fveclib=amdlibm
error: invalid value 'amdlibm' in '-fveclib=amdlibm'

$ clang -c prog.c -fveclib=AMDLIBM
clang: error: unsupported option 'AMDLIBM' for target 'aarch64'