posix: signal: use the provided argument in the macros

The `SIGNO_WORD_IDX` & `SIGNO_WORD_BIT` macros should have
used its own argument `_signo` instead of `signo`. It didn't
cause and error because the function's argument has `signo`.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>