RFR 8004128: NPG: remove stackwalking in Threads::gc_prologue and gc_epilogue code

[Coleen Phillimore]

Summary: remove bcx and mdx handling.  We no longer have to convert 
bytecode pointers or method data pointers to indices for GC since 
Metadata aren't moved.

Tested with nsk.quick.testlist, jck tests, JPRT.

Most of this is renaming bcx to bcp and mdx to mdp.  The content changes 
are in frame.cpp.  StefanK implemented 90% of these changes.

open webrev at http://cr.openjdk.java.net/~coleenp/8004128/
bug link https://bugs.openjdk.java.net/browse/JDK-8004128

Thanks,
Coleen