Implement IRQ software support for RISC-V.

Implement IRQ software support for RISC-V.

Well, at least PicoRV32-specific. Turns out there is no RISC-V
specification for simple microcontroller-like interrupts, so PicoRV32
implements its' own based on custom opcodes.

It's somewhat esoteric, and for example doesn't offer a global interrupt
enable/disable. For this we implement a thin wrapper in assembly and
then expose it via a few helpers in irq.h.