XS RFR: 8014460 Need to check for non-empty EXT_LIBS_PATH before using it

[David Holmes]

Trivial fix for the open arm.make files - we should only update the LIBS 
variable when EXT_LIBS_PATH is defined.

webrev:
http://cr.openjdk.java.net/~dholmes/8014460/webrev/

Pushing to hotspot-emb.

Testing: local builds with/without EXT_LIBS_PATH set; jprt ARM builds

Thanks,
David