10: these raw instructions are unsafe r=dvc94ch a=strake

`wfi` is safe, but the rest are not. Let's make them all unsafe until we have a better idea — the return types are wrong anyhow.


Co-authored-by: M Farkas-Dyck <strake888@gmail.com>

9: use `NonZeroUsize` where appropriate r=dvc94ch a=strake



Co-authored-by: M Farkas-Dyck <strake888@gmail.com>

8: mepc is word-size r=dvc94ch a=strake



Co-authored-by: M Farkas-Dyck <strake888@gmail.com>

In the original riscv-rust fork the target arch was simply named
"riscv", but RISC-V support landed in Rust with "riscv32" as the arch
name instead.

Include "riscv64" optimistically for future-proofing.