When an access-to-subprogram type is qualified as not null, then the
compiler is likely to generate a more efficient code. This patch adds
such qualifiers to the internal runtime bodies, so no clients of those
runtime units are affected.

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

gcc/ada/

	* libgnat/g-alleve.adb (Bit_Operation): Now a not-null type.
	* libgnat/g-sechas.adb (Fill_Buffer_Access): Likewise.
	* libgnat/s-dwalin.adb (Callback): Likewise.