build: avoid passing kill empty input in Makefile

Using `xargs -r` on some platforms and `xargs` on others doesn't work,
we can't guarantee whether xargs is GNU or not. Avoid the issue by only
running kill if there are processes to clean.

PR-URL: https://github.com/nodejs/node/pull/12158


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>