cmd/compile: provide Add/Cas/Exchange atomic intrinsics on riscv64

Provide Add32, Add64, Cas32, Cas64, Exchange32 and Exchange64 atomic
intrinsics on riscv64.

Updates #36765

Change-Id: I9a3b7d2ce3d49f699171fd76a0fed891d149a6bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/223559


Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>