Avoid exception propagation during bootstrap

Avoid exception propagation during bootstrap

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.

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