[Ada] Remove old vxworks6 from Makefile.rtl

[Ada] Remove old vxworks6 from Makefile.rtl

Pre vxworks7 code excepting legacy vxworks6 code is removed from
Makefile.rtl and unused files are deleted.

gcc/ada/

	* Makefile.rtl (*vxworks*): Remove most pre-vxworks7 code.
	* vxworks-arm-link.spec: Remove.
	* vxworks-e500-link.spec: Likewise.
	* vxworks-smp-arm-link.spec: Likewise.
	* vxworks-smp-e500-link.spec: Likewise.
	* vxworks-smp-x86-link.spec: Likewise.
	* libgnat/system-vxworks-arm-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks-arm-rtp.ads: Likewise.
	* libgnat/system-vxworks-arm.ads: Likewise.
	* libgnat/system-vxworks-e500-kernel.ads: Likewise.
	* libgnat/system-vxworks-e500-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks-e500-rtp.ads: Likewise.
	* libgnat/system-vxworks-x86-kernel.ads: Likewise.
	* libgnat/system-vxworks-x86-rtp-smp.ads: Likewise.
	* libgnat/system-vxworks-x86-rtp.ads: Likewise.