First, install XCode Tools. Then install MacPorts, and use it to install: gcc and binutils.
sudo port -v selfupdate
sudo port install i386-elf-gcc i386-elf-binutils
Then install qemu, either via macports (sudo port install qemu) or homebrew. (Homebrew lacks the packaged cross compiler.)
tar xfz gdb-7.8.tar.gz
cd gdb-7.8
./configure --target=i386-elf --disable-werror --program-prefix=i386-elf-
make
sudo make install
git clone git://scriptroute.cs.umd.edu/geekos-project