RFR (S): 8006965: test_gamma should run with import JDK

[Christian Thalinger]

http://cr.openjdk.java.net/~twisti/8006965

8006965: test_gamma should run with import JDK
Reviewed-by:

Right now test_gamma runs with the boot JDK which is JDK n-1 (where
JDK n is the version we are actually compiling for).  This setup is
unsupported and thus should not be done during HotSpot builds.

The fix is to always use JDK_IMPORT_PATH instead of JAVA_HOME when
running test_gamma.

make/bsd/makefiles/buildtree.make
make/defs.make
make/linux/makefiles/buildtree.make
make/solaris/makefiles/buildtree.make