Add mkspecs for 32-bit Mac OS X builds.

Add mkspecs/macx-clang-32
Add mkspecs/macx-g++-32
Add mkspecs/common/clang-mac.conf

Skip the rest of the Mac mkspecs (icc, llvm, g++40,
g++42, builder, Xcode, xlc) pending a cleanup where
we determine which ones to keep.

Change-Id: I00ceddfcbdb5119cb132433bad43e4aed016eab2
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>