riscv: cmpxchg: Remove unnecessary definitions of cmpxchg & xchg

The custom xchg/cmpxchg_release macro definitions have no
difference from the common code from the binary view. The
xchg32/64 macro definitions have been abandoned in Linux. Thus,
remove all of them.

This is a preparation for the next cmpxchg_small & xchg8 patches.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>