* Don't erase __attribute__ under clang-cl
The shim fires when __GNUC__ is absent, which clang-cl does not define even
though it supports __attribute__ fully. Erasing it breaks clang's own intrinsics
headers: mmintrin.h declares __m64 as a vector via __attribute__((vector_size)),
so it degrades to a scalar and every use of it fails to compile.
* Don't erase __attribute__ under clang-cl
clang-cl supports it but does not define __GNUC__, so the shim fired and broke
clang's own intrinsics headers.
David Racine