Add exclusion between instrumentation and GC

Instrumentation calls VisitClasses while exclusive holding the
mutator lock. This can cause deadlocks since VisitClasses needs to
decode JNI weak globals. If the system weak slow path flag is set,
then we wait holding locks while exclusive holding the mutator lock.
This causes a deadlock since the GC cannot acquire the mutator lock
to sweep system weaks.

This fixes a deadlock seen in one of the tracing tests.

Change-Id: I580152118e068a70f309dcc19df4144afec835dd