Check alignment of fp in unwind_frame

A misaligned frame pointer is certainly not a valid frame pointer and
with strict alignment enabled (as on CHERI) can cause panics when it is
loaded from later in the code.

This is a recommit of 40e0fa10f58d90744c2857b57adf0ddbce1a1e1c with
is_aligned() corrected to __is_aligned().

Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D34646