gen_makefile: don't shell out to nproc

Use python's multiprocessing.cpu_count() instead of "$(nproc)". The
latter didn't work properly in Makefiles, the right syntax would have
been "$(shell nproc)". Make didn't complain about it and assumed that we
want to use all CPUs with an empty argument "-j ", but meson doesn't
accept this syntax.

Change-Id: I58ca082339f3aff813f587f4c2be9c0951b9b2dd
1 file changed