aarch64: correctly handle zero-sized bit-fields in AAPCS64 [PR102024]

On aarch64 the AAPCS64 states that an HFA is determined by the 'shape' of
the object after layout has been completed, so anything that adds no
members and does not cause the layout to be modified should be ignored
for the purposes of determining which registers are used for parameter
passing.

A zero-sized bit-field falls into this category.  This was not handled
correctly for C structs and in G++-11 only handled correctly because
such fields were eliminated early by the front end.

gcc/ChangeLog:

	PR target/102024
	* config/aarch64/aarch64.cc (aapcs_vfp_sub_candidate): Handle
	zero-sized bit-fields.  Detect cases where a warning may be needed.
	(aarch64_vfp_is_call_or_return_candidate): Emit a note if a
	zero-sized bit-field has caused parameter passing to change.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/aapcs64/test_28.c: New test.