(XS) RFR: 8020799 Allow customization of hotspot source directories and files

[David Holmes]

This is an enabler for the co-existence of our closed ppc sources and 
the incoming ppc64 port. We need to be able to customize the source 
directories and the excluded sources lists.

We use the usual mechanism of -including the same named makefile from 
the HS_ALT_MAKE directory. This one is a little unusual in that the 
place where we do the -include is very, very specific. We have to do it 
after the default setup of the sources/excludes and before that 
information is used to generate the set of source files to compile.

webrev: http://cr.openjdk.java.net/~dholmes/8020799/webrev.v3/

Pushing to hotspot-emb

Thanks,
David