`riscv`: align assembly functions with `cortex-m` crate

[romancardenas]

Motivated by #179 (and #180), I took a look on how the cortex-m crate deals with assembly instructions, and most of them are safe and use options to avoid the compiler from using the stack etc.

@rust-embedded/riscv what do you think about updating our assembly functions to follow the same fashion as in cortex-m? @rust-embedded/cortex-m any advice on this?

[thejpster]

In general I like it when the risc-v ecosystem kinda looks and feels like the cortex-m ecosystem, as it makes it easier to move between the two.