[Ada] Combine system.ads file - vxworks7 kernel constants.

Systemitize Word_Size and Memory_Size declarations rather than hard code
with numerical values or OS specific Long_Integer size.

gcc/ada/

	* libgnat/system-vxworks7-aarch64.ads (Word_Size): Compute
	based on Standard'Word_Size. (Memory_Size): Compute based
	on Word_Size.
	* libgnat/system-vxworks7-arm.ads: Likewise.
	* libgnat/system-vxworks7-e500-kernel.ads: Likewise.
	* libgnat/system-vxworks7-ppc-kernel.ads: Likewise.
	* libgnat/system-vxworks7-ppc64-kernel.ads: Likewise.
	* libgnat/system-vxworks7-x86-kernel.ads: Likewise.
	* libgnat/system-vxworks7-x86_64-kernel.ads: Likewise.