RFR: 8166188: G1 Needs pre barrier on dereference of weak JNI handles

[Kim Barrett]

Here's the updated webrevs, incorporating feedback received so far.

full: http://cr.openjdk.java.net/~kbarrett/8166188/hotspot.05/
incr: http://cr.openjdk.java.net/~kbarrett/8166188/hotspot.05.inc/

For x86 and ARM I added resolve_jobject functions to their respective
macroAssembler files, and call those to generate the return object
handling code.  Merging the two cases for Sparc is harder and I'm
going to leave it to someone else.  I'm also leaving any such merging
for other (non-Oracle) ports to their respective maintainers.

Rewrote the error checking in SignatureChekker::check_obj to simplify
the control flow and provide more information.  Rather than
conditionally selecting between ReportJNIFatalError or assert to
report problems, instead just always use guarantee.  Recall that we
only get here if CheckJNICalls or in a debug build.  This made
SignatureChekker::_thread unused, so removed it.