riscv: use simplest asm expression when possible

Let's take advantage of assembler pseudoinstructions:

- convert `addi rd, rs, 0` to `mv rd, rs`
- convert `jal x0, somewhere` to `j somewhere`
- convert `csrrs x0, csrreg, rs` to `csrs csrreg, rs`
- convert `fscsr x0, rs` to `fscsr rs`

And simplify zero offsets to simply 0.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>