Print registers on an arm64 spinlock data abort
When checking if the spinlock count is correct in a data abort we can
panic without printing the registers. These are useful to debug the
abort, e.g. by giving the fault address register.
Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D40228
(cherry picked from commit 0731b0a9f1f9e3b4ec699ec67b6eb24159fa9b05)