build: support 64 bits darwin builds

Before this commit, gyp built 32 bits versions of libuv exclusively.

Enable with:

  $ ./gyp_uv -Dtarget_arch=x64

common.gypi

@@ -172,6 +172,14 @@
             '-Wno-unused-parameter',
           ],
         },
+        'conditions': [
+          ['target_arch=="ia32"', {
+            'xcode_settings': {'ARCHS': ['i386']},
+          }],
+          ['target_arch=="x64"', {
+            'xcode_settings': {'ARCHS': ['x86_64']},
+          }],
+        ],
         'target_conditions': [
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},