Merge c2f9885a8aa3a820eefdacccf3fcc6b9d87e3284 into 18.x

[brad0]

[SPARC] Support reserving arbitrary general purpose registers
(#74927)

This adds support for marking arbitrary general purpose registers -
except for those with special purpose (G0, I6-I7, O6-O7) - as reserved,
as needed by some software like the Linux kernel.

[brad0]

/cherry-pick c2f9885

[llvmbot]

/pull-request #81397

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Brad Smith (brad0)

[SPARC] Support reserving arbitrary general purpose registers (#74927)

This adds support for marking arbitrary general purpose registers -
except for those with special purpose (G0, I6-I7, O6-O7) - as reserved,
as needed by some software like the Linux kernel.