Reduce stack usage for ISRs; minor fixes.

Reduce stack usage for ISRs; minor fixes.

Don't back up all registers on isr handlers - they don't read/modify
    them.  This saves stack space.
extended_bios_data_area_s must be packed to match ebda spec.
Enable irqs on int 08 - follows old bochs bios code.
Fix bug in int 76 -- should clear disk_interrupt_flag not
    floppy_harddisk_info.
Make sure we alert in disk_ret on failure case.
int 18/19 entry points need to setup cld/%ds too.
asm in handle_1587 clobbers flags - note that in clobber list.