Fix a crash on fork when using a custom memory allocator (ex: using
PYTHONMALLOC env var).

_PyGILState_Reinit() and _PyInterpreterState_Enable() now use the
default RAW memory allocator to allocate a new interpreters mutex on
fork.
(cherry picked from commit 5d926471)
Co-authored-by: Victor Stinner <victor.stinner@gmail.com>