Only smp runtimes are built for vxworks7*, even though the -smp suffix
is removed during install. Therefore, in general, the build macros for
the non-smp runtimes are superfluous except on the legacy ppc-vxworks6
target where both the smp and non-smp runtime are built.  Lastly, an
error message is added if a runtime build is commanded that doesn't
exist, rather then letting the build mysteriously fail.

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

gcc/ada/

	* Makefile.rtl [arm,aarch64 vxworks7]: Remove rtp and kernel
	build macros and set an error variable if needed.
	[x86,x86_vxworks7]: Likewise.
	[ppc,ppc64]: Set an error variable if needed.
	(rts-err): New phony Makefile target.
	(setup-rts): Depend on rts-err.