ada: Pass -msmp when linking for ppc-vx6 --RTS=rtp-smp

ada: Pass -msmp when linking for ppc-vx6 --RTS=rtp-smp

gprbuild and gnatmake won't pass --RTS=rtp-smp to the compiler driver
for linking.  The flag was not used during linking: the .spec files
named as linker options were all we passed for the linker to get the
-L flags for lib_smp and lib.

There was a problem, though: although /lib_smp/ and /lib/ were to be
searched in this order, and the specs files did that correctly, the
compiler would search /lib/ first regardless, because
STARTFILE_PREFIX_SPEC said so, and specs files cannot override that.

With this patch, we make sure the rtp-smp runtime causes -msmp to be
added to the command line passed to the compiler driver for linking,
and a corresponding patch for the ppc-vxworks configuration makes the
GCC compiler driver use this flag to select /lib_smp/ rather than
/lib/.

gcc/ada/

	* libgnat/system-vxworks-ppc-rtp-smp.ads: Add -msmp to
	Linker_Options pragma.