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

[Tim Bell]

Hi David:

> 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 build

Looks good from the build side.

Tim