This package does not contain the sources used to build the resulting packages. 
It build-depends on the gcc source and uses that (via the
gcc-<ver>-source package)

All the supported gcc cross-targets are built by this package.

To change the set of TARGET architectures built or the set of HOST architectures
they are built for, set the TARGET_LIST and HOST_LIST variables respectively,
and generate the source trees. For instance, to build armhf, armel
cross-compilers for amd64, i386 and arm64 you can generate the trees with

 TARGET_LIST="armel armhf" HOST_LIST="amd64 i386 arm64" ./gensources

This is the host-list that's normally built in Debian. This generates a separate
source package for each target. At this point each of these source packages is
ready to be built normally with dpkg-buildpackage on a system where the target
architecture is enabled as a multiarch architecture.

The ./buildall script automates the process of building the full set
for target architectures locally. It needs sudo, devscripts and equivs
installed, and <target>-binutils packages available.

You can build a specific target like this:
TARGET_LIST=armhf PARALEL=8 ./buildall