[compiler-rt] [compiler-rt] Fix SetAlternateSignalStack return type for GCC 15.2.0 (PR #202094)

[via llvm-commits]

https://github.com/clearnature commented:

[compiler-rt] Fix SetAlternateSignalStack return value, drop style-only changes
- Return oldstack.ss_sp instead of nullptr when alt stack already set
- Pass altstack_base parameter to altstack.ss_sp (was ignored)
- Revert void* → void * whitespace changes (this directory uses
  Google style per .clang-format; formatting is separate concern)

Fixes build failure with GCC 15.2.0 on Ubuntu 26.04.

https://github.com/llvm/llvm-project/pull/202094