Fix platform detection for intel platforms

Installation of numpy fails on my Mac OSX machine as `distutils/system_info.py` wrongly assumes a non-intel platform:

    >>> from distutils.util import get_platform
    >>> get_platform()
    'macosx-10.8-x86_64'

This PR fixes the issue by testing for:

    'x86_64' in get_platform()

Related: #2631