build,win: set /MP separately in Debug and Release

Setting /MP globally causes it to appear twice in the command line due
to a GYP bug, which causes the project to be rebuilt unconditionally due
to an msbuild bug.

PR-URL: https://github.com/nodejs/node/pull/16415
Fixes: https://github.com/nodejs/node/issues/16367


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>