Document how to run fulltest under valgrind on non-amd64

We only run ntruprime-fulltest with valgrind on amd64 because it takes too long for debci. Upstream only supports binaries where fulltest passed under valgrind. We should document some procedure to get fulltest invoked on the other architectures too. Some ideas:

• Simple README.Debian with step-by-step instructions how to download, extract and invoke fulltest under valgrind on other architectures. The GCC compile farm provides hardware that can be used, but we don't have root so instructions has to work with that.

• I have GitLab runners for ppc64el and arm64el that could be added to the Salsa libntruprime project which could run the testing. Probably a new debian/salsa-ci.yml job has to be added for this, it could be relatively simple.

Of course, platforms where valgrind doesn't work on aren't possible to test this way.