... when its length is close to Natural'Last.

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

gcc/ada/

	* libgnat/a-strunb__shared.ads (Allocate): Additional parameter
	to provide additional amount of space to be allocated.
	* libgnat/a-strunb__shared.adb (Aligned_Max_Length): Limit
	length to Natural'Last when requested length is larger than it.
	(Allocate): Merge two slightly different implementations into
	one.