python: make it cross-compile (almost)

It builds, but fails to install:

/nix/store/xdswdgz11w9lm9n9bzb6spl7dj45s2x2-coreutils-8.21/bin/install -c ./Lib/plat-linux2/regen /nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/plat-linux2
[3s[a/nix/store/xdswdgz11w9lm9n9bzb6spl7dj45s2x2-coreutils-8.21/bin/install -c -m 644 ./LICENSE /nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/LICENSE.txt
[3s[aif test -d /nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/distutils/tests; then \
	/nix/store/xdswdgz11w9lm9n9bzb6spl7dj45s2x2-coreutils-8.21/bin/install -c -m 644 ./Modules/xxmodule.c \
		/nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/distutils/tests ; \
fi
[3s[aPYTHONPATH=/nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7   \
	_PYTHON_PROJECT_BASE=/tmp/nix-build-python-2.7.7-arm-unknown-linux-gnueabi.drv-0/Python-2.7.7 _PYTHON_HOST_PLATFORM=linux2-arm PYTHONPATH=/tmp/nix-build-python-2.7.7-arm-unknown-linux-gnueabi.drv-0/Python-2.7.7/build/lib.linux2-arm-2.7:./Lib:./Lib/plat-linux2 python -Wi -tt /nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/compileall.py \
	-d /nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7 -f \
	-x 'bad_coding|badsyntax|site-packages|lib2to3/tests/data' \
	/nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7
Traceback (most recent call last):
  File "/nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/compileall.py", line 16, in <module>
    import struct
  File "/nix/store/7viy33b43wrxd30idbpjl213s0yadc7a-python-2.7.7-arm-unknown-linux-gnueabi/lib/python2.7/struct.py", line 1, in <module>
    from _struct import *
ImportError: /tmp/nix-build-python-2.7.7-arm-unknown-linux-gnueabi.drv-0/Python-2.7.7/build/lib.linux2-arm-2.7/_struct.so: wrong ELF class: ELFCLASS32
make: *** [libinstall] Error 1
[q[q[qbuilder for `/nix/store/n7kkvfdxfb04g1wrgfq8bqivdfx5fkaf-python-2.7.7-arm-unknown-linux-gnueabi.drv' failed with exit code 2
error: build of `/nix/store/n7kkvfdxfb04g1wrgfq8bqivdfx5fkaf-python-2.7.7-arm-unknown-linux-gnueabi.drv' failed