[libunwind] libunwind: Do not use __attribute__((target("gcs"))) with non-clang … (PR #138077)

[Andrew Pinski via cfe-commits]

pinskia wrote:

Actually GCC 15 does support GCS, the syntax is just `__attribute__((target("+gcs")))` Note the + there. The error message is clear too.

https://github.com/llvm/llvm-project/pull/138077