Recognize rtems as an o/s.

From-SVN: r12399

gcc/config.sub

@@ -662,7 +662,7 @@ case $os in
 	      | -lynxos* | -bosx* | -nextstep* | -cxux* | -aout* | -elf* \
 	      | -ptx* | -coff* | -ecoff* | -winnt* | -domain* | -vsta* \
 	      | -udi* | -eabi* | -lites* | -ieee* | -go32* | -aux* \
-	      | -cygwin32* | -pe* | -psos* | -moss* | -proelf* )
+	      | -cygwin32* | -pe* | -psos* | -moss* | -proelf* | -rtems*)
 	# Remember, each alternative MUST END IN *, to match a version number.
 		;;
 	-sunos5*)