RFR: JDK-8213338 Reduce the number of generated make targets

[Magnus Ihse Bursie]

On some systems, we're starting to hit limits due to the number of 
generated make targets. With the recent change from run-test-* to 
test-*, where the run-test-* still were kept for backwards 
compatibility, we effectively increased the number of make targets.

This also goes for exploded-run-test-* and exploded-test-*, however, 
using the exploded testing is a less commonly used method, and can be 
used without direct make targets. I propose to remove those targets to 
lessen the pressure. All exploded tests can still be run as before, but 
using "make exploded-test TEST=foo" instead of "make exploded-test-foo".

The one exception is exploded-test-gtest, which is commonly used by 
Hotspot developers to test Hotspot without having to build a complete 
jdk-image before testing. I've kept this, along with the mechanism for 
adding more targets for exploded-test-* on a case-by-case basis, should 
the need arise.

Bug: https://bugs.openjdk.java.net/browse/JDK-8213338
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8213338-reduce-number-of-generated-make-targets/webrev.01

/Magnus