riscv: s64ilp32: Enable native atomic64

The traditional rv32 Linux (s32ilp32) uses a generic version of the
lib/atomic64.c, which are inaccurate atomic64 primitives and couldn't
co-work with READ_ONCE/WRITE_ONCE,  atomic_8/16/32. The s64ilp32 could
use native AMO instructions to implement accurate atomic64 primitives.

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