buildbot/compile.py needs to have an option to use fewer cores

[jeffhammond]

The compiler.py script should support an argument to limit build parallelism. I attempted to run this script on my Raspberry Pi 4 and it maxed out the CPU temperature and ultimately bricked the device (resolved with a power-cycle).

I try to work on a patch but would be very happy to see someone else do it first.

import multiprocessing
...
DEFAULT_CPU_COUNT = 4
...
def do_compile(args):
    try:
        cpu_count = multiprocessing.cpu_count()
    except NotImplementedError:
        cpu_count = DEFAULT_CPU_COUNT