Omit platform-specific assembler source from builds as needed

The default filename rules do not match .S or .asm, so the
platform-specific assembler implementations of CaptureContext() were not
being affirmatively excluded from other platforms’ builds. This
previously worked without causing problems because the Mac build
environment didn’t know what to do with .asm files, and the Windows
build environment didn’t know what to do with .S files. Now that another
platform that may understand .S files is being added, the rules for when
to build these files must be tailored a bit more tightly.

BUG=crashpad:30

Change-Id: Ib62e619c007320d45279c104b3e229d92698aa72
Reviewed-on: https://chromium-review.googlesource.com/406348
Reviewed-by: Robert Sesek <rsesek@chromium.org>

client/client.gyp

@@ -61,6 +61,11 @@
             ],
           },
         }],
+        ['OS!="mac"', {
+          'sources!': [
+            'capture_context_mac.S',
+          ],
+        }],
       ],
       'direct_dependent_settings': {
         'include_dirs': [

util/util.gyp

@@ -284,6 +284,10 @@
               },
             }],
           ],
+        }, {  # else: OS!="win"
+          'sources!': [
+            'win/capture_context.asm',
+          ],
         }],
       ],
     },