[TESTING]: boot2 compilation with -mregparm=3

Hi,

This patch:


        http://lev.vlakno.cz/~rdivacky/boot2-mregparm.patch


makes boot2 to be compiled with -mregparm=3 (which shrinks it
some). It changes CFLAGS to include -mregparm=3 and also
rewrites sio.S to use register passing.

This survived my testing in qemu and jhb_at_s review.

Please test this patch with boot2 booting via serial console,
and with gptboot/zfsboot using serial console.

Thank you! This is the last FreeBSD patch in the clang-boot2
saga so please help.

roman