llvm: doesn't actually need pythonnative

llvm: doesn't actually need pythonnative

LLVM doesn't actually need pythonnative, the host Python2 is sufficient but
cmake's executable searching currently isn't reliable in cross-compilations.

Convince cmake by setting PYTHON_EXECUTABLE to point at python2 in the HOSTTOOLS
directory.  Note that currently LLVM *needs* python2 currently:

https://github.com/llvm-mirror/llvm/blob/master/CMakeLists.txt#L670

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>