ia32: Fix alignment of _Atomic fields [PR65146]

ia32: Fix alignment of _Atomic fields [PR65146]

For _Atomic fields, lowering the alignment of long long or double etc.
fields on ia32 is undesirable, because then one really can't perform atomic
operations on those using cmpxchg8b.

The following patch stops lowering the alignment in fields for _Atomic
types (the x86_field_alignment change) and for -mpreferred-stack-boundary=2
also ensures we don't misalign _Atomic long long etc. automatic variables
(the ix86_{local,minimum}_alignment changes).
Not sure about iamcu_alignment change, I know next to nothing about IA MCU,
but unless it doesn't have cmpxchg8b instruction, it would surprise me if we
don't want to do it as well.
clang apparently doesn't lower the field alignment for _Atomic.

2020-08-27  Jakub Jelinek  <jakub@redhat.com>

	PR target/65146
	* config/i386/i386.c (iamcu_alignment): Don't decrease alignment
	for TYPE_ATOMIC types.
	(ix86_local_alignment): Likewise.
	(ix86_minimum_alignment): Likewise.
	(x86_field_alignment): Likewise, and emit a -Wpsabi diagnostic
	for it.

	* gcc.target/i386/pr65146.c: New test.