RFR: JDK-8201236 Straighten out dtrace build logic

Fri Apr 6 10:57:01 UTC 2018

The dtrace build logic was copied straight out of the old Hotspot build 
system, and is quite convoluted.

It should be split into the separate parts it actually contains of:
1) A gensrc step which runs with other gensrc ahead of compilation
2) Two independent libraries that can be build at any time
3) Two special dtrace-generated .o files that must be linked with the JVM

I have also cleaned up includes in generateJvmOffsets.cpp.

I have verified with COMPARE_BUILD that no changes has happened in any 
native library.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201236
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8201236-straighten-out-dtrace/webrev.01

/Magnus