Make javac_opts of java_common.compile expandable

[iirina]

The javac_opts argument of java_common.compile doesn't accept expandable locations right now. Something like $(GENDIR)/$(location :stub) should be supported (the same as is supported in javacopts of java_library).

[lberki]

We have the ctx.expand_location() function which does the exact same thing. You can call it from anywhere you can call java_common.compile().