MFC r354240, r354252, r358906

libi386/comconsole.c updates:

We don't support configuring serial PCI cards in EFI. Make this clearer in the
source rather than obfuscaring it behind NO_PCI (nothing else declares that,
so it's not making the ifdefs clearer).

libi386/comconsole.c cstyle cleanup

test if port does exist via using scratch register