[RFC] RISC-V: Add proposed Ztso atomic mappings

[Andrea Parri]

On Fri, May 05, 2023 at 10:12:56AM -0700, Patrick O'Neill wrote:
> The RISC-V Ztso extension currently has no effect on generated code.
> With the additional ordering constraints guarenteed by Ztso, we can emit
> more optimized atomic mappings than the RVWMO mappings.
> 
> This patch implements Andrea Parri's proposed Ztso mappings ("Proposed
> Mapping").
>   https://github.com/preames/public-notes/blob/master/riscv-tso-mappings.rst
> 
> LLVM has implemented this same mapping (Ztso is still behind a
> experimental flag in LLVM, so there is *not* a defined ABI for this yet).
>   https://reviews.llvm.org/D143076

Given the recent patches/discussions, it seems worth pointing out the
the Ztso mappings referred to above was designed to be compatible with
the mappings in Table A.6 and that they are _not_ compatible with the
mappings in Table A.7 or with a "subset" of A.7 (even assuming RVTSO).

  Andrea