tests: Fix sscanf integer tests

Drop the double tests that klibc doesn't care about
and that where hence failing in the first place.

Check the returned integer values too.

Fixes Bug: https://bugs.gentoo.org/show_bug.cgi?id=464908

Reported-by: Mike Pagano <mpagano@gentoo.org>
Signed-off-by: maximilian attems <max@stro.at>
README.klibc: update build information

It seems this file was still conveying the old way to build klibc
and hence confusing the users, simplify and reword build instructions.

Signed-off-by: maximilian attems <max@stro.at>