doc: adding ARCHS flag to OS X build command

Adding further instructions about building for OS X, as recent versions of xcodebuild will automatically build for i386 (can someone say, "monoculture") even on x86_64 machines! Specifically including instructions about including the "ARCHS" flag to specify the right architecture.

This is trivial, but will save people time.
Merge branch 'master' of https://github.com/nathanjsweet/libuv