Use x86-interrupt calling convention rather than naked fns

[hawkw]

When rust-lang/rust#39832 is merged to nightly, I can rewrite my interrupt-handling code for x86 to use the x86-interrupt calling convention, which will be much less ugly than the current implementation and might also lead to improved performance.

This may also allow some interrupt handlers to be written more generally, for both 32-bit and 64-bit x86?

[hawkw]

x86-interrupt ABI should be on nightly rust now, so this can be started...