[clang] [llvm] [RISC-V]Implement -m{,no}fence-tso (PR #151638)

[Liao Chunyu via llvm-commits]

ChunyuLiao wrote:

Indeed, as jrtc27 said, GCC already supports this. 

There are cases where the firmware cannot be upgraded on certain hardware, so it might make sense for us to provide an option to accommodate these systems.

When the hardware does not support fence.tso, fence rw, rw may faster because fence.tso will be handled via a trap and software emulation.

https://github.com/llvm/llvm-project/pull/151638