Clarify '/CETCOMPAT' is only for x86/x64

This follows up to my previous commit [1] for google#835.

This option (/CETCOMPAT) is not compatible with ARM64 builds.

As a preparation to build Mozc executables for ARM64 (google#1130) let's
specify it in 'x86_Base' and 'x64_Base' instead of globally defining it.

There must be no change in the final artifacts.

 [1]: a0133fb

src/gyp/common_win.gypi

@@ -135,6 +135,9 @@
             'UACExecutionLevel': '0',  # level="asInvoker"
             'UACUIAccess': 'false',    # uiAccess="false"
             'MinimumRequiredVersion': '10.0',
+            'AdditionalOptions': [
+              '/CETCOMPAT',
+            ],
           },
         },
         'msvs_configuration_attributes': {
@@ -158,6 +161,9 @@
           },
           'VCLinkerTool': {
             'TargetMachine': '<(win_target_machine_x64)',
+            'AdditionalOptions': [
+              '/CETCOMPAT',
+            ],
           },
         },
       },
@@ -338,9 +344,6 @@
           'user32.lib',
           'uuid.lib',
         ],
-        'AdditionalOptions': [
-          '/CETCOMPAT',
-        ],
         'DataExecutionPrevention': '2',        # /NXCOMPAT
         'EnableCOMDATFolding': '2',            # /OPT:ICF
         'GenerateDebugInformation': 'true',    # /DEBUG