Serial debugging code must not access BDA.

Serial debugging code must not access BDA.

Regular serial writing code uses the system timer to timeout failed
    writes - however, serial debugging can't rely on access to the BDA
    segment or the hardware timer.
Therefore, implement a simple debug only serial writing function and
    separate it from the regular serial output code.
Also include change to dump_regs - don't call __dprintf if debugging
    not on.