This commit brings the natmod tests in the CI build process for the RV32
platform. Not all example natmods are tested at the moment, as
`features` requires soft-float support, and `btree` needs thread-local
storage support in `mpy_ld.py` when built with the CI's toolchain.
Co-authored-by: Damien George <damien@micropython.org>
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>