arm64: fix db_read_bytes() for size == 8

There is a mistake in the cast, resulting in a truncated read to tmp64.
Switch from int to uint64_t, and adjust the other casts for clarity.

Add a comment explaining why we do this at all.

Reported by: dfr
Reviewed by: dfr, mmel, emaste, jhb (all a previous version)
PR: 276406
Fixes: a67687fcd8f5 ("Use native-sized accesses when accessing memory from kdb")
Differential Revision: https://reviews.freebsd.org/D43479

(cherry picked from commit 9c2e1a54f71a399fc4645c4b8bed044705629143)