[Ada] Avoid exception propagation during bootstrap

[Eric Botcazou]

This addresses PR ada/100486, which is the bootstrap failure of GCC 11 for 32-
bit Windows in the MSYS setup.  The PR shows that we cannot rely on exception 
propagation being operational during the bootstrap, at least on the 11 branch, 
so fix this by removing the problematic raise statement.

Tested on x86-64/Linux, applied on the mainline and 11 branch.


2021-10-20  Arnaud Charlet  <charlet@adacore.com>

	PR ada/100486
	* sem_prag.adb (Check_Valid_Library_Unit_Pragma): Do not raise an
	exception as part of the bootstrap.

-- 
Eric Botcazou
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