* config.sub (s390, s390x): Use 'ibm' vendor default, but allow

overrides.

Previously, 'ibm' was forced, which caused problems when someone
wanted to use 'busybox' as a vendor and cross-compile with a slightly
different toolchain. The fix changed behaviour such that without any
vendor, it would use to 'unknown' rather than 'ibm' as before.

This patch aims to compromise between the old and new behaviour by
making 'ibm' a default for those CPUs when no vendor is specified, but
if the user explicitly provides a vendor that is used instead. This
sort of "vendor defaulting" has plenty of precedent in config.sub, so
it seemed like a good approach.

Signed-off-by: Ben Elliston <bje@gnu.org>

config.sub

@@ -1763,6 +1763,9 @@ case $vendor in
 			*-os400*)
 				vendor=ibm
 				;;
+			s390-* | s390x-*)
+				vendor=ibm
+				;;
 			*-ptx*)
 				vendor=sequent
 				;;