libuv: Restore support for setting child process affinity

libuv now prefers `posix_spawn` to launch processes on unix.
It does not offer a way to set the child process affinity on
startup, so fall back to `fork` and `exec` if we need affinity.

This restores commit 24de561a1a (libuv: unix,win: add uv_spawn option
to set child CPU affinity mask, 2017-09-06, v3.12.0-rc1~412^2~2).