Prevent races with GC when transferring objects between threads

We could have races with the GC when JVMTI code transfers a local
reference from one thread to another. This race would happen if a GC
was currently underway causing an unmoved reference to be transferred
to a thread that has already fixed up all of its references. This
meant that the receiving thread would now have a reference belonging
to the from-space on its stack.

This could cause memory errors and CHECK failures.

To fix this we make sure to run all checkpoints where local references
might be transferred in a GC-critical-section.

We also needed to fix a too-strict check in the method verifier where
we were incorrectly asserting that suspension be allowable when in
fact it was not necessary in all cases.

Test: ./test.py --host -j50
Bug: 67838964

Change-Id: Ib9166cc233d3c1b488864c0aff33246a3f99e436

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