MIPS: cmpxchg: Implement __cmpxchg() as a function

MIPS: cmpxchg: Implement __cmpxchg() as a function

Replace the macro definition of __cmpxchg() with an inline function,
which is easier to read & modify. The cmpxchg() & cmpxchg_local() macros
are adjusted to call the new __cmpxchg() function.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16353/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>