This moves the registry higher in the LLVM library dependency stack.
Every client of the target registry needs to link against MC anyway to
actually use the target, so we might as well move this out of Support.
This allows us to ensure that Support doesn't have includes from MC/*.
Differential Revision: https://reviews.llvm.org/D111454
Add `-ffixed-a[0-6]` and `-ffixed-d[0-7]` and the corresponding
subtarget features to prevent certain register from being allocated.
Differential Revision: https://reviews.llvm.org/D102805
