[Ada] Fix internal error on pragma Machine_Attribute with string constant

[Eric Botcazou]

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.

Tested on x86-64/Linux, applied on the mainline.


2025-10-26  Eric Botcazou  <ebotcazou@adacore.com>

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


2025-10-26  Eric Botcazou  <ebotcazou@adacore.com>

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

-- 
Eric Botcazou
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