The 'ARCHS' key should be an array.

The ninja generator only supports setting the 'ARCHS' key as array, and
ninja does not support '$(ARCHS_STANDARD_64_BIT)' too. The 'x86_64' is
accepted by both ninja and xcodebuild, and is also used by node.
Merge pull request #43 from brightray/arches-key

The 'ARCHS' key should be an array.