[X86][MC] Support encoding of EGPR for APX (#71909)

#70958 adds registers R16-R31 (EGPR), this patch
1. Introduces a new instruction prefix REX2
2. Supports encoding of EGPR with REX2 for legacy instructions in MAP
0/1
3. Supports encoding of EGPR with EVEX for the existing instructions in
EVEX space

RFC:

https://discourse.llvm.org/t/rfc-design-for-apx-feature-egpr-and-ndd-support/73031/4