amd64 db_trace: Reject unaligned frame pointers

Switch to using db_addr_t to hold frame pointer values until they are
verified to be suitably aligned.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D41532

(cherry picked from commit 4a9cd9fc22d7f87a27ccd3d41b93a0356cd7061c)