[clang] [M68k] Change gcc register name from a7 to sp. (PR #87095)

[Jim Lin via cfe-commits]

tclin914 wrote:

> Is it possible use `TargetInfo::getGCCRegAliases` to model the aliasing between a7 and sp? Also, could you add a simple test?

Implement getGCCRegAliases and add testcase.

https://github.com/llvm/llvm-project/pull/87095