Move td_sigqueue to the end of struct thread

In order to preserve KBI in stable branches, replace the existing
td_sigqueue slot with padding and move the expanded (as of r315949)
td_sigqueue to the end of the struct.

Reported by: jhb
Suggested by: kib
Reviewed by: jhb, kib, vangyzen
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D10515