[8u60] RFR: 8072740: move closed jvm.cfg files out of open repo

[David Holmes]

bug: https://bugs.openjdk.java.net/browse/JDK-8072740

webrev: http://cr.openjdk.java.net/~dholmes/8072740/webrev.8u/

Simple fix, contributed by Dean Long, that allows the jvm.cfg file to be 
located in the "closed" location. The arm and ppc jvm.cfg files can then 
be moved to that "closed" location.

The backport is slightly different to the JDK 9 fix as the inclusion of 
the "custom" file at the end, rather than the beginning, precludes 
defining the ALT_JVMCFG_SRC variable there.

I intend to push this to hs-dev rather than jdk8u-dev as there is other 
work going into hs-dev that is waiting for this.

Thanks,
David