Ada: Fix internal error on pragma Machine_Attribute with string constant

Ada: Fix internal error on pragma Machine_Attribute with string constant

This was reported a long time ago and is a fairly pathological case,
so the fix is purposely ad hoc: when the attribute name of a pragma
Machine_Attribute is not a string literal, its processing needs to
be delayed for the back-end.

gcc/ada/
	PR ada/13370
	* sem_prag.adb (Analyze_Pragma) <Pragma_Machine_Attribute>: Set the
	Has_Delayed_Freeze flag if the argument is not a literal.

gcc/testsuite/
	* gnat.dg/machine_attr3.ads, gnat.dg/machine_attr3.adb: New test.