RFR 8140685: Fix backtrace building to not rely on constant pool merging

[Coleen Phillimore]

Summary: Save Throwable in a list which is a weak GC root for walking 
during MetadataOnStackMark

This was the original implementation that I had for finding Method* in 
backtraces but it was slow because I had used jweak to save the 
Throwable object.   Using internal vm weak GC roots, this does not cause 
a performance regression in refWorkload.  This fix is more 
straightforward and does not rely on the constant pool index of the 
merged constant pool during redefinition to find the method's name.
This is one fix that enables removing merged constant pools during 
redefinition (work in progress).  It also is would be used as a model 
for JEP 259: Stack Walking API
https://bugs.openjdk.java.net/browse/JDK-8043814.

The code that registers MemberNames for replacing Method* during 
redefinition may need to use the same mechanism for performance, if 
MemberName arrays are used for the Stack Walking API.

I assume this will generate comments from the GC group.

bug link https://bugs.openjdk.java.net/browse/JDK-8140685
local webrev at http://javaweb.us.oracle.com/~cphillim/webrev/8140685.01

Tested with RBT quick (former "quick" tests), jdk/java/lang/instrument 
tests and the test that I wrote for handling backtraces with 
redefinition in hotspot/test/runtime/RedefineTests.

Thanks,
Coleen