dev-python/reedsolo: Add reedsolo for host

This will be used to calculate dm-verity Forward Error Correction
(FEC) bits for images.

Changes from upstream:
- Downgrade EAPI to 7
- Remove dependency on cython (not required for host builds)

BUG=b:438865027
TEST=emerge dev-python/reedsolo

Change-Id: If65a921316727439475d0563a586236241e322e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/7262924
Commit-Queue: Sarthak Kukreti <sarthakkukreti@google.com>
Tested-by: Sarthak Kukreti <sarthakkukreti@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>