after the boot block changes on i386, sthen noticed a dmesg change

after the boot block changes on i386, sthen noticed a dmesg change
-pci0 at mainbus0 bus 0: configuration mode 1 (bios)
+pci0 at mainbus0 bus 0: configuration mode 1 (no bios)

deraadt then spent many hours narrowing down the problem to the inline
assembly in pciprobe().

It tried to save the carry flag result of pci bios present.  But did so
after a shift which sets the carry flag.  Ask for CF in the output and
avoid the shift.

ok deraadt@ kettenis@