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: nodejs/node#16415
Fixes: nodejs/node#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>

common.gypi

@@ -125,6 +125,7 @@
             'BasicRuntimeChecks': 3, # /RTC1
             'AdditionalOptions': [
               '/bigobj', # prevent error C1128 in VS2015
+              '/MP', # compile across multiple CPUs
             ],
           },
           'VCLinkerTool': {
@@ -180,6 +181,9 @@
             'EnableFunctionLevelLinking': 'true',
             'EnableIntrinsicFunctions': 'true',
             'RuntimeTypeInfo': 'false',
+            'AdditionalOptions': [
+              '/MP', # compile across multiple CPUs
+            ],
           },
           'VCLibrarianTool': {
             'AdditionalOptions': [
@@ -212,9 +216,6 @@
         # and their sheer number drowns out other, more legitimate warnings.
         'DisableSpecificWarnings': ['4267'],
         'WarnAsError': 'false',
-        'AdditionalOptions': [
-          '/MP', # compile across multiple CPUs
-        ],
       },
       'VCLibrarianTool': {
       },