util: use c11 alignas instead of rolling our own

Due to how alignas is defined, it itsn't allowed to use it on a struct,
it needs to be used on the first member instead. So move the declaration
in those cases.

This still leaves the ALIGN16 macro using compiler-specific directives,
because it's a lot of work to untangle the above. This probably deserves
its own MR.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16908>

src/util/macros.h

@@ -413,6 +413,7 @@ u_uintN_max(unsigned bit_size)
 #ifndef __cplusplus
 #ifdef _MSC_VER
 #define alignof _Alignof
+#define alignas _Alignas
 #else
 #include <stdalign.h>
 #endif