RFR 8u backport: 8077608: [TESTBUG] Enable Hotspot jtreg tests to run in agentvm mode

[Stuart Monteith]

Hello,
  Currently the jdk8u codebase fails some JTreg Hotspot tests when
running in the -agentvm mode. This is because the ProcessTools class
is not passing the classpath. There are substantial time savings to be
gained using -agentvm over -othervm.

Fortunately, there was a fix for jdk9 (8077608) that has not been
backported to jdk8u.  The details are as follows:

http://mail.openjdk.java.net/pipermail/hotspot-dev/2015-April/017937.html
https://bugs.openjdk.java.net/browse/JDK-8077608
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/af2a1e9f08f3

The patch just needed a slight change, to remove the change to the
file "test/compiler/uncommontrap/TestUnstableIfTrap.java" as that test
doesn't exist on jdk8u.

My colleague Ningsheng has kindly hosted the change here:

    http://cr.openjdk.java.net/~njian/8077608/webrev.00


BR,
   Stuart