riscv: align assembly functions with cortex-m

[romancardenas]

Closes #181

On instructions that trigger exceptions

I decided to keep instructions that trigger exceptions (e.g., ecall) unsafe for their potential side effects. For example, if no exception handler is defined, triggering an exception can cause the program to crash. Also, raising exceptions is a potential way to avoid critical sections. Thus, one must be careful with them.

In any case, I'm open to to discuss this in case anyone has a different point of view.

[rmsyn]

I decided to keep instructions that trigger exceptions (e.g., ecall) unsafe for their potential side effects. For example, if no exception handler is defined, triggering an exception can cause the program to crash.

I think this is the right decision, too. Even if there is an exception handler defined, it could be defined using unsafe Rust, C, or assembly. Inherently unsafe, since we can't guarantee at compile-time what exception-triggering code calls into.