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

[Ada] Combine system.ads file - vxworks7 rtp 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-rtp-smp.ads (Word_Size):
	Compute based on Standard'Word_Size. (Memory_Size): Compute
	based on Word_Size.
	* libgnat/system-vxworks7-arm-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-e500-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-e500-rtp.ads: Likewise.
	* libgnat/system-vxworks7-ppc-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-ppc-rtp.ads: Likewise.
	* libgnat/system-vxworks7-ppc64-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-x86-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks7-x86-rtp.ads: Likewise.