Previously this was listed as a suppression, but we actually want this
permanently unsanitized. This should be faster and more reliable since
TASN won't have to match against symbolicated backtraces.

This introduces a new method to encapsulate checking whether the current
Ractor owns the vm->ractor.sync lock. This allows us to disable TSan on
it since that operation should be safe, and still get validation of
other uses.

* Add gc_enable/disable to TSAN suppressions
* Remove deleted methods from tsan suppressions
* Remove TSAN errors we've fixed
* Add another two inline cache suppressions
* Improve comments

Although it almost certainly works in this case, volatile is best not
used for multi-threaded code. Using atomics instead avoids warnings from
TSan.

This also simplifies some logic, as system_working was previously only
ever assigned to 1, so --system_working <= 0 should always return true
(unless it underflowed).