Cross-compilation on Mac OSX currently doesn't work. While building
stage 1, the build system uses the `ar` for the target architecture
instead of the `ar` for build/host architecture.

The cause is a hack added in 24746fe7
(2010), to supporting bootstrap compilers built with older versions of
Xcode. Xcode 4.3 started installing command line tools in a different
location. Assuming this all behind us now, and the paths didn't change
again (you never now), we can delete the hack.

Deleting the hack fixes the cross compilation issue. Tested by Trac user
jakzale.

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1231