Workaround Python multiprocessing pool limitation to fix parallel test suite on Windows.

[juj]

Even latest Python 3.11 still has an issue that it cannot spawn a multiprocessing pool with > 61 processes on Windows. python/cpython#89240

Cap worker count to 61 on Windows to work around.

[kripken]

How about just making get_num_cores return a value capped to 61 on windows? That seems simpler, and more future-proof.

[juj]

I thought about that at first, but that would pessimize other places that call get_num_cores(), e.g.

the manual multiprocessing spawner that I wrote to avoid multiprocessing.Pool() bugs at https://github.com/emscripten-core/emscripten/pull/19216/files#diff-4b34c94c6049a1b42f714f19c01fd70da33949b91e332e0ec0fb41711d75c537L200 .

Looks like also e.g. ninja builder invokes that

def run_ninja(build_dir):
  diagnostics.warning('experimental', 'ninja support is experimental')
  cmd = ['ninja', '-C', build_dir, f'-j{shared.get_num_cores()}']

and it probably does not have a problem with parallelizing to > 61 cores.