ada: Handle GNATcheck violations

ada: Handle GNATcheck violations

For the GNATcheck rule "Improper_Returns", either use pragma Annotate
to exempt the violation with the rationale "early returns for performance",
or refactor the code by replacing multiple returns by a single return
statement with a conditional expression; this is more readable and
maintainable, and also conformant with a Highly Recommended design principle
of ISO 26262-6.  For the GNATcheck rule "Discriminated_Records", use pragma
Annotate to exempt the violation with the rationale "only variant records
are disallowed".

gcc/ada/

	* libgnarl/a-reatim.adb (Time_Of): Add pragma to exempt
	Discriminated_Records.
	* libgnat/s-imguti.adb (Round, Set_Decimal_Digits): Likewise.
	* libgnat/s-multip.adb (Number_Of_CPUs): Likewise.
	* libgnarl/s-tpopsp__posix-foreign.adb (Self): Refactor multiple
	returns.