ada: Add guard to System.Val_Real.Large_Powfive against pathological input

There is no need to keep multiplying the result once it saturates to +Inf.

gcc/ada/ChangeLog:

	* libgnat/s-powflt.ads (Maxpow_Exact): Minor comment fix.
	* libgnat/s-powlfl.ads (Maxpow_Exact): Likewise.
	* libgnat/s-powllf.ads (Maxpow_Exact): Likewise.
	* libgnat/s-valrea.adb (Large_Powfive) [1 parameter]: Exit the loop
	as soon as the result saturates to +Inf.
	(Large_Powfive) [2 parameters]: Likewise.