riscv: s64ilp32: Implement cmpxchg_double

The s64ilp32 has the ability to exclusively load and store (ld/sd)
a pair of words from an address. Then the SLUB can take advantage
of a cmpxchg_double implementation to avoid taking some locks.

This patch provides an implementation of cmpxchg_double for 64-bit
pairs, and activates the logic required for the SLUB to use these
functions (HAVE_ALIGNED_STRUCT_PAGE and HAVE_CMPXCHG_DOUBLE).

Similar commit: 5284e1b4 ("arm64: xchg: Implement
cmpxchg_double")

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