util: use c11 alignas instead of rolling our own

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>