kboot: Avoid UB in signed shift

offset is signed. Copy it to the unsigned res before shifting. This
avoids any possible undefined behavior for right shifting signed
numbers. No functional change intended (and the code generated is the
nearly same for aarch64).

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D44285

(cherry picked from commit 8b1925f29c54f5791db3c8dcdf2b67541bb8ab32)