Reduce stack usage of hw irq handlers.

Reduce stack usage of hw irq handlers.

Avoid using call16_int() -- it consumes too much stack space.
    Instead, use a new function (call16_simpint).  This assumes that
    the handler wont corrupt regs - which should be a safe assumption,
    because if they did corrupt regs they wouldn't work on any bios.
Avoid enabling irqs in the hw irq handlers - there are no loops in the
    handlers that could cause any notable latency.