headers: Avoid builtin clash with _xgetbv in Clang 9

Clang 9 adds _xgetbv for non-msvc targets, by adding a builtin
named __builtin_ia32_xgetbv and a define in a header redirecting
_xgetbv to __builtin_ia32_xgetbv. The define redirection breaks
the intrin-impl.h code already on the prototype.

Signed-off-by: Martin Storsjö <martin@martin.st>