change mk_add_options MOZ_MAKE_FLAGS="-j5" to mk_add_options MOZ_MAKE_FLAGS="-j$(sysctl -n hw.ncpu)" on macos

[k2662]

we should change

mk_add_options MOZ_MAKE_FLAGS="-j5"

to

mk_add_options MOZ_MAKE_FLAGS="-j$(sysctl -n hw.ncpu)"

[Alex313031]

@k2662 Good idea. You can make a PR if you want, or I could change it. Note that the mozconfig files for cross-compiling on linux for mac should be left as is, or for an equivalent, use $NPROC