Build boot2 with -mregparm=3, ie. pass upto 3 arguments via registers.
This modifies CFLAGS and tweaks sio.S to use the new calling convention.
The sio_init() and sio_putc() prototypes are modified so that other
users of this code know the correct calling convention.

This makes the code smaller when compiled with clang.

Reviewed by: jhb
Tested by: me and Freddie Cash <fjwcash gmail com>