[llvm] [MC][TableGen] Make MCRegisterClasses relocation-free (PR #206753)

[Alexis Engelke via llvm-commits]

aengelke wrote:

The bigger challenge after this is going to be moving TargetRegisterClass to .rodata, which has a pointer to the MCRegisterClass. Would it be acceptable to put these in the same .cpp file/data structures? Otherwise not sure how to change that... TargetRegisterClass combined are currently our biggest .data.rel.ro member with 138560 for *RegClass and 17320 for *RegisterClasses, so I think we should do *something* here.

https://github.com/llvm/llvm-project/pull/206753