build system: tighten down .xcompile handling some more

Bail out if .xcompile is incomplete or can't be regenerated.

Change-Id: I74adeded7a3e849b25bf65c5b02f67820f29c7e2
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/12477
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

util/xcompile/xcompile

@@ -355,3 +355,4 @@ for architecture in $SUPPORTED_ARCHITECTURES; do
 		report_arch_toolchain
 	fi
 done
+echo XCOMPILE_COMPLETE:=1