MPI implementations intercepting Signals is incompatible with Julia GC safepoint

Thanks again for your help with https://github.com/JuliaParallel/MPI.jl/issues/720 - this one is unrelated (except that issue #720 lead us to create more comprehensive unit test revealing this new, probably unrelated segfault).

**Summary of this problem:** a segfault occurs when GC is triggered in a multithreaded+MPI context.

**How to reproduce:** I have create a draft PR adding a GC.gc() call in one of MPI.jl's existing multithreaded test: see PR Request https://github.com/JuliaParallel/MPI.jl/pull/724

The draft PR is based off the most recent commit where all tests passed (Tag 0.20.8). In the output of "test-intel-linux", the salient output is

```
signal (11): Segmentation fault
in expression starting at /home/runner/work/MPI.jl/MPI.jl/test/test_threads.jl:18
ijl_gc_enable at /cache/build/default-amdci4-2/julialang/julia-release-1-dot-8/src/gc.c:2955
```

The change we made is in the file test/test_threads.jl, where we added the following if clause:

```
Threads.@threads for i = 1:N
reqs[N+i] = MPI.Irecv!(@view(recv_arr[i:i]), comm; source=src, tag=i)
reqs[i] = MPI.Isend(@view(send_arr[i:i]), comm; dest=dst, tag=i)
if i == 1
GC.gc()
end

end
```

We experience similar problems with MPICH 4.0 in our package (https://github.com/Julia-Tempering/Pigeons.jl), but not with MPICH 4.1.

**Related discussions**

- https://juliaparallel.org/MPI.jl/stable/knownissues/#Multi-threading-and-signal-handling

This describes a similar issue in the context of UCX. However this problem does not seem limited to UCX from our investigations so far.

- https://github.com/JuliaParallel/MPI.jl/issues/337

This describes a similar issue in the context of OpenMPI. However it seems that certain versions of MPICH and intel MPI (which is MPICH-derived) might suffer from a similar issue?

In light of these two sources, perhaps other environment variables in the style of https://github.com/JuliaParallel/MPI.jl/blob/6d513bb37da76182f2c31baff44f1f53a103295d/src/MPI.jl#L133 could be set to address this issue? I was wondering if anyone might have some suggestion on whether that's a reasonable hypothesis? Having limited MPI experience I am not sure what these environment variables might be.

Thank you so much for your time.