shells/mksh: fix crash on 16-CURRENT

Kernel now reports support for 126 signals, while the libc headers only
report 32. This mismatch caused an out-of-bounds write initialising
internal tables.

Reported by: hackee <h4ckee@proton.me>
PR: 292146
MFH: 2025Q4

(cherry picked from commit 1c257703ed4c59a485b28b17c7a1541b0c203191)