Add support for arch specific port_sources
Add support for arch specific port_sources

On one project I have a need to specify port_sources on R14 only
and on another different project port_sources for Darwin and Linux.
To this end add support to handle tuples of the form
{ArchRegex, PortSource} in the port_sources list, eg:

    {port_sources, [{"R14", ["c_src/*.c"]}]}.

mbbx6spp/rebar