cmd/compile: actually intrinsify runtime/internal/atomic.{And,Or}{8,} on RISCV64

Actually enable intrinsics for runtime/internal/atomic.{And,Or}{8,} on RISCV64.
This seems to have been lost when CL 268098 was rebased.

Change-Id: If072daa79c8964b186c127d5e065a7cc9e23ba27
Reviewed-on: https://go-review.googlesource.com/c/go/+/302229


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>