Merge #99

99: Remove uses of unstable features and always use inline assembly r=Disasm a=taiki-e

Currently, the `inline-asm` feature uses unstable features, but by replacing the uses of asm_const with `concat!`+`stringify`, the uses of unstable features can be removed.

https://github.com/rust-embedded/riscv/blob/cd31989ba11d5d64e1addd8aab98bfb00dd927d5/src/lib.rs#L17-L18



Co-authored-by: Taiki Endo <te316e89@gmail.com>