[Ada] Relax null exclusion mismatch check in Relaxed_RM_Semantics mode

[Pierre-Marie de Rodat]

This is useful for e.g. CodePeer usage.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch6.adb (Null_Exclusions_Match): Relax null exclusion
	mismatch check when Relaxed_RM_Semantics is set.
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