Remove mips MD atomic_load_64 and atomic_store_64.

The only users of the functions were db_read_bytes() and
db_write_bytes() ddb(4) interfaces. Replace the calls with direct
reads and writes, which are automatically atomic on 64bits and n32.

Note that removed assembler implementation for mips32 is not atomic
anyway.

Reviewed by: jhb
Discussed with: imp
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13586