python3-cython: add patch to prefix map source paths in generated files

python3-cython: add patch to prefix map source paths in generated files

Cython generates C source code that contains mentions to the original
.py files, which results in build paths being embedded inside the
binaries.

Implement prefix remapping to change these build paths to point at the
target debug directory, so that we don't leak build paths and have
reproducible builds.

This patch is currently not submitted upstream, but will be shortly and
I expect it to evolve before being merged.

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