[Linux] Prevent GC from running during process teardown

[d-netto]

Context

We send a signal 15 to shutdown our servers.

We noticed that some of our servers that receive the termination signal are segfaulting in GC, which leads to false alarms in our internal monitors that track GC-related crashes.

Hypothesis

We suspect this pathological case may be happening:

• Process receives signal 15, which is captured by the signal listener thread.
• Signal listener initiates process' teardown (e.g. through raise).
• IIRC such operation is not atomic in Linux, i.e. the kernel will gradually kill the threads, but it's possible for us to spent a few ms in a state where part of the threads in the system are alive, and part have already been killed (this point needs some confirmation).
• With part of the process alive, and part of the process dead, we try to enter a GC, see a bunch of Julia data structures in an intermediate/corrupted state, which leads us to crash when running the GC.

Mitigation

Since our main goal is to get rid of the GC crashes that happen around server shutdown, we believe that it would be sufficient to just prevent the last bullet point. I.e. we prevent the system from even running a GC when we're about to kill the process, and we wait for any ongoing GC to finish.

Co-debugged with @kpamnany.