[Ada] Pragma Atomic wrongly rejected on composite component

[Eric Botcazou]

> That regresses on gnat.dg/specs/atomic1.ads for aarch64/-mabi=ilp32,
> missing the error on line 13.  The missing error on line 9 is
> preexisting.

That's sort of expected, since the point of the patch is to make the 2 
situations equivalent wrt atomicity.  Clearly the test is not portable and 
will fail on targets for which pointer size and word size are not equal.

-- 
Eric Botcazou