[Mips] support "sp" named register

After Linux kernel commit
commit 200ed341b864 ("mips: Implement "current_stack_pointer"")
We observe the following build error when compiling the Linux kernel
targeting Mips:
fatal error: error in backend: Invalid register name global variable

Fixes: https://github.com/llvm/llvm-project/issues/54174
Link: https://github.com/ClangBuiltLinux/linux/issues/1608

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D120926