Adding an attempt to crossbuild bootstrap tools for the pi.

Based on the cross/make-bootstrap-tools.nix I had for the sheevaplug
in my nixos.org svn configuration directory.
Removing any cross-building kind of native build for the pi.
gcc/gcc-cross-wrapper: making gcc47 work with uclibc

It wants pthreads, and libpthreads wants libdl, and this can
only be found with -rpath-link when cross building.
Setting gcc47 for the cross build tools
gcc47: trying to fix cross-building a gcc
gcc47: fix on configureFlags, for cross-built gcc
Typo in 4.7 builder.sh
Fixing the bootstrap-tools-crosspi for gcc47
Improving the bootstrap tools (missing libs)
Restoring the gcc 4.6 builder. No need to change it.
I forgot to undo a gcc46 unneeded change.
Undoing glibc 2.13 work for the pi

(previous attempt of bootstrapping from the armv5tel bootstrap tools)
Undoing more glibc unneeded changes
Removing a glibc outdated mention in stdenvLinux
Adding the armv6l bootstrap; I forgot that file in previous commits.
Going with armv6l-linux bootstrap-tools.

I had lost this change in some branch.
gcc4.7: making the cross-built gcc respect withAbi, withArch, ...
The glibc doesn't like "gnueabihf" in the triplet.

I better set "gnueabi".
Setting a public URL for the pi bootstrap tools
gcc46: moving the bootstrap logic into nix

It was in the builder.sh.