This commit lets the compiler know about interrupts and allows
optimizations to be performed based on that: interrupts are eliminated
when they appear to be unused in a program. This is done with a new
pseudo-call (runtime/interrupt.New) that is treated specially by the
compiler.
This brings a big speedup. Not counting gofmt time,
`make gen-device-avr` became about 3x faster. In the future, it might be
an idea to generate the AST in-memory and write it out already
formatted.