Fix mips build after introduction of MD definitions of atomic_load_64

Fix mips build after introduction of MD definitions of atomic_load_64
and atomic_store_64.

The MD definitions are provided for LP64 only, while mips also uses
them for 32bit and n32.  Only define mips variants for 32bit and n32
and change the syntax to match common definitions.

Note that this commit does not fix 32bit asm implementation to follow
new KBI, this will be fixed later.  The functions are only used for 8
byte ddb accesses so the known bug does not prevent normal kernel
operations.

Sponsored by:	The FreeBSD Foundation