cmd/internal/obj/riscv: remove ACFLWSP/ACFSWSP and ACFLW/ACFSW
These are RV32-only instructions that will not be implemented.
Updates #71105
Change-Id: Ie386fe36e56f1151bb8756088dd79804584317c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/702395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com> Reviewed-by: Mark Ryan <markdryan@rivosinc.com> Reviewed-by: Mark Freeman <markfreeman@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>