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
Change-Id: I051bf8dcf2801d9671cf83f0e0a94e1f19b98c0f
1 file changed