[clang] [clang] Implement gcc_struct attribute on Itanium targets (PR #71148)

[Yonggang Luo via cfe-commits]

lygstate wrote:

mesa also need this
```
./../src/compiler/glsl_types.c:749:8: warning: unknown attribute 'gcc_struct' ignored [-Wunknown-attributes]
  749 | struct PACKED explicit_matrix_key {
      |        ^~~~~~
../../src/util/macros.h:214:35: note: expanded from macro 'PACKED'
  214 | #    define PACKED __attribute__((gcc_struct,__packed__))
```

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