aarch64: Don't trust TYPE_ALIGN for pointers [PR108910]

aarch64: Don't trust TYPE_ALIGN for pointers [PR108910]

The aarch64 PCS rules ignore user alignment for scalars and
vectors and use the "natural" alignment of the type.  GCC tried
to calculate that natural alignment using:

  TYPE_ALIGN (TYPE_MAIN_VARIANT (type))

But as discussed in the PR, it's possible that the main variant
of a pointer type is an overaligned type (although that's usually
accidental).

This isn't known to be a problem for other types, so this patch
changes the bare minimum.  It might be that we need to ignore
TYPE_ALIGN in other cases too.

gcc/
	PR target/108910
	* config/aarch64/aarch64.cc (aarch64_function_arg_alignment): Do
	not trust TYPE_ALIGN for pointer types; use POINTER_SIZE instead.

gcc/testsuite/
	PR target/108910
	* gcc.dg/torture/pr108910.c: New test.