The current configuration of the QNX and VxWorks runtimes does not support
atomic primitives.

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

gcc/ada/

	* libgnat/system-qnx-arm.ads: Set Support_Atomic_Primitives to
	false.
	* libgnat/system-vxworks7-aarch64.ads: Ditto.
	* libgnat/system-vxworks7-aarch64-rtp-smp.ads: Ditto.
	* libgnat/system-vxworks7-arm.ads: Ditto.
	* libgnat/system-vxworks7-arm-rtp-smp.ads: Ditto.
	* libgnat/system-vxworks7-x86-kernel.ads: Ditto.
	* libgnat/system-vxworks7-x86-rtp-smp.ads: Ditto.