More JIT debug data synchronisation.

I want to be able to reason about consistency of the data even
when it is being modified (e.g. debug-malloc backtrace running
on one thread while the JIT is running on a different thread).

Test: testrunner.py --host -t 137

(cherry picked from commit d767f2df)

Bug: 73127105
Bug: 74361929
Change-Id: Ibd3a2eead9836d3f2700334abb9e02ce0df8ac6e