This feature is an architecture feature, not an OS feature, so enable
on vx7r2 for arm and aarch64 to coincide with what is done on similarly
capable targets.

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

gcc/ada/

	* libgnat/system-vxworks7-arm.ads (Support_Atomic_Primitives):
	Set True.
	* libgnat/system-vxworks7-arm-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-aarch64.ads: Likewise.
	* libgnat/system-vxworks7-aarch64-rtp-smp.ads: Likewise: