[clang] [clang] Stub out gcc_struct attribute (PR #71148)

[John McCall via cfe-commits]

rjmccall wrote:

> @rjmccall Re asking GCC developers about gcc_struct on non-x86: https://gcc.gnu.org/pipermail/gcc/2024-January/243154.html. Either GCC devs aren't really worried about this or I can't properly write emails (what's totally possible).

Alright, well, we tried.  I think rolling forward with what your proposal on non-x86 platforms is reasonable.

https://github.com/llvm/llvm-project/pull/71148