RFR: JDK-8243988 Added flexibility in build system for unusal hotspot configurations

[Magnus Ihse Bursie]

This is the changes to the build system that was proposed as part of 
JDK-8240224. This will allow jtreg to be run with a custom JVM 
configuration, and makes the semantic distinction between JAVA_FLAGS and 
BUILDJDK_JAVA_FLAGS. If also updates the exclude pattern for serialgc 
since the files they point to have been removed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8243988
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8243988-build-changes-for-no-serialgc/webrev.01

/Magnus