libc: minimal: ctype: express ctype limits in more direct fashion

Based on review feedback, it was suggested to express ctype
character checks in a more direct fashion, rather than using
arithmetic, and allow the compiler to optimize as it sees fit.

https://github.com/zephyrproject-rtos/zephyr/pull/99451#\
discussion_r2530339430

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>