Mac OS: support Python >= 3.8 by using fork-based multiprocessing (#18124)

As detailed in https://bugs.python.org/issue33725, starting new
processes with 'fork' on Mac OS is not guaranteed to work in general.
As of Python 3.8 the default process spawning mechanism was changed
to avoid this issue.

Spack depends on the fork-based method to preserve file descriptors
transparently, to preserve global state, and to avoid pickling some
objects. An effort is underway to remove dependence on fork-based
process spawning (see #18205). In the meantime, this allows Spack to
run with Python 3.8 on Mac OS by explicitly choosing to use 'fork'.

Co-authored-by: Peter Josef Scheibel <scheibel1@llnl.gov>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>
Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

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