build,win: use /MP for debug builds

PR-URL: #16333
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>

common.gypi

@@ -167,9 +167,6 @@
             'EnableFunctionLevelLinking': 'true',
             'EnableIntrinsicFunctions': 'true',
             'RuntimeTypeInfo': 'false',
-            'AdditionalOptions': [
-              '/MP', # compile across multiple CPUs
-            ],
           },
           'VCLibrarianTool': {
             'AdditionalOptions': [
@@ -202,6 +199,9 @@
         # and their sheer number drowns out other, more legitimate warnings.
         'DisableSpecificWarnings': ['4267'],
         'WarnAsError': 'false',
+        'AdditionalOptions': [
+          '/MP', # compile across multiple CPUs
+        ],
       },
       'VCLibrarianTool': {
       },