xcompile: Don't warn on missing power8 compiler

Until there's a reason to, don't print a warning about the missing
power8 compiler.

Change-Id: I47c60e0a16892f0fa228e1439e0424926bca00a4
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12634
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>

util/xcompile/xcompile

@@ -351,7 +351,7 @@ test_architecture() {
 		CLANG="clang -target ${clang_arch}-${TABI} -ccc-gcc-name ${GCC}"
 	fi
 
-	if [ -z "$GCC" -a -z "$CLANG" ]; then
+	if [ -z "$GCC" -a -z "$CLANG" -a "power8" != "$architecture" ]; then
 		echo "Warning: no suitable compiler for $architecture." >&2
 		return 1
 	fi