Fix a dead lock between garbage colletion and sample-based tracing.

We have been observing some rare deadlock in run-test 099-vmdebug
between
- a thread doing garbage collection, which had disabled weak
  references access and was trying to (re-)acquire the mutator
  lock;
- a sampling profiling thread used in sample-based tracing, which
  had acquired the mutator lock exclusively and was blocking on a
  condition variable regarding weak references access.

This change prevents garbage collection from occurring when
sampling the thread stacks in order to avoid this deadlock.

Test: for i in $(seq 1 10); do art/test/testrunner/testrunner.py --ndebuggable --debuggable --host --64 --verbose --debug --ndebug --gcstress -t 099-vmdebug; done
Bug: 73624630
Change-Id: I73272c2b1d7aa2554a88bb128d3646193bd1db04