Darwin, PPC: Fix struct layout with pragma pack [PR110044].

Darwin, PPC: Fix struct layout with pragma pack [PR110044].

This bug was essentially that darwin_rs6000_special_round_type_align()
was ignoring externally-imposed capping of field alignment.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
	PR target/110044

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (darwin_rs6000_special_round_type_align):
	Make sure that we do not have a cap on field alignment before altering
	the struct layout based on the type alignment of the first entry.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/darwin-abi-13-0.c: New test.
	* gcc.target/powerpc/darwin-abi-13-1.c: New test.
	* gcc.target/powerpc/darwin-abi-13-2.c: New test.
	* gcc.target/powerpc/darwin-structs-0.h: New test.

(cherry picked from commit 84d080a29a780973bef47171ba708ae2f7b4ee47)