Introduce a setting to control the toolset for which d8 is compiled

src/d8.gyp

Index: src/d8.gyp
diff --git a/src/d8.gyp b/src/d8.gyp
index bc1dedca0fe0bf6e5ba282d0d1e10dd3302f6ef1..0e51baaacac06825ba682ccfbb3b8407af0fff47 100644
--- a/src/d8.gyp
+++ b/src/d8.gyp
@@ -32,6 +32,7 @@
     # Enable support for Intel VTune. Supported on ia32/x64 only
     'v8_enable_vtunejit%': 0,
     'v8_enable_i18n_support%': 1,
+    'v8_toolset_for_d8%': 'target',
   },
   'includes': ['../build/toolchain.gypi', '../build/features.gypi'],
   'targets': [
@@ -49,6 +50,9 @@
         'd8.cc',
       ],
       'conditions': [
+        [ 'want_separate_host_toolset==1', {
+          'toolsets': [ '<(v8_toolset_for_d8)', ],
+        }],
         [ 'console=="readline"', {
           'libraries': [ '-lreadline', ],
           'sources': [ 'd8-readline.cc' ],