Apple project generator respects default cxx platform

Created by: ghvg1313

Resolves https://github.com/facebook/buck/issues/2032 using a different approach.

This PR essentially solve two issues:

1. supported_platforms_regex now won't be included in the undesired platform, originally this could create an empty project target or unsupported source/header being included.
2. Header collision by glob usage in exported_platform_headers, this is covered in detail in https://github.com/facebook/buck/issues/2032

@williamtwilson While i don't think this is a very elegant solution it does the job. Since this reverted couple of commits (bd07dcd6, 4ab71d6a and abb6939c) and uses a different approach than excluding headers based on platform, I'm up for discussion