mac: Support ARCHS with zero or one element in the make and ninja generators.

test/mac/archs/test-archs-x86_64.gyp

Index: test/mac/archs/test-archs-x86_64.gyp
===================================================================
--- test/mac/archs/test-archs-x86_64.gyp	(revision 0)
+++ test/mac/archs/test-archs-x86_64.gyp	(revision 0)
@@ -0,0 +1,23 @@
+{
+ 'targets': [
+  {
+   'target_name': 'lib',
+   'product_name': 'Test64',
+   'type': 'static_library',
+   'sources': [ 'my_file.cc' ],
+   'xcode_settings': {
+     'ARCHS': [ 'x86_64' ],
+   },
+  },
+  {
+   'target_name': 'exe',
+   'product_name': 'Test64',
+   'type': 'executable',
+   'dependencies': [ 'lib' ],
+   'sources': [ 'my_main_file.cc' ],
+   'xcode_settings': {
+     'ARCHS': [ 'x86_64' ],
+   },
+  },
+ ]
+}