[ARM] Support fp16/bf16 using t constraint

fp16 and bf16 values can be used in GCC's inline assembly using the "t"
constraint, which means "VFP floating-point registers s0-s31" - fp16 and
bf16 values are stored in S registers too.

This change ensures that LLVM is compatible with GCC for programs that
use fp16 and the 't' constraint.

Fixes #57753

Differential Revision: https://reviews.llvm.org/D134553