linux: Also translate the signal if the code is CLD_KILLED

This fixes ./waitid.gen.test from the strace(1) test suite.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32617

(cherry picked from commit c8c93b151678b57d86cd50509c9cbb863bbe9e57)