build: use $(RM) in Makefile for consistency

Also allows someone to reassign `$RM`, e.g. with `RM=rm -v` instead of
`rm -f` (the default) should they want to. We're currently using a
mixture of `$(RM)` and `rm -f`.

There are a couple of places which aren't doing -f, have them do it for
consistency.

PR-URL: https://github.com/nodejs/node/pull/12157


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>