[libc] clean up ctype negative handling

The ctype functions will sometimes be passed negative numbers, such as
EOF. Some of the previous implementations didn't handle these properly.
Additionally, the tests did not check any negative numbers. These
problems have been fixed.

This patch fixes https://github.com/llvm/llvm-project/issues/62000

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D147813