Change p4conf.rb to only build 64bit on Mac OS.
Previously, P4Ruby was built as a fat universal lib;
however, as of 12.1 we will no longer build a universal
API for Mac. Although Ruby on Mac is a universal
binary, it seems happy enough to load 64bit only
libraries.
Also re-added rule to link against 'p4sslstub', which
I inadvertently removed in change 408875.
User visible change to be documented in release notes.