RISC-V: Check for hardware TSO support

With the Ztso extension [1], some hardware will support TSO on RISC-V. That allows us to reduce the generation of memory fences, given the stronger memory model compared to RVWMO.

[1] https://github.com/riscv/riscv-isa-manual/blob/6dcbc6da9ada01f0f57da83cda6059bdec57619f/src/ztso-st-ext.adoc#L1