ada: Fix ancient bug in pragma Suppress (Alignment_Check)

ada: Fix ancient bug in pragma Suppress (Alignment_Check)

The recent change that streamlined the implementation of alignment checks
has uncovered an ancient bug in the implementation of pragma Suppress on
a specific object:

  pragma Suppress (Alignment_Check, A);

The pragma would work only if placed before the address clause:

  A : Integer;
  pragma Suppress (Alignment_Check, A);
  for A'Address use ...

but not if placed (just) after it:

  A : Integer;
  for A'Address use ...
  pragma Suppress (Alignment_Check, A);

which seems unfriendly at best.

gcc/ada/ChangeLog:

	* sem_prag.adb (Analyze_Pragma) <Process_Suppress_Unsuppress>: For
	Alignment_Check on a specific object with an address clause and no
	alignment clause, toggle the Check_Address_Alignment flag present
	on the address clause.