bios_emulator: Allow x86 to use the emulator

There is an implicit assumption that x86 machines want to use raw I/O in the
BIOS emulator, but this should be selectable. Add an CONFIG_X86EMU_RAW_IO
option to control it instead.

Also fix a few bugs which cause warnings on x86 and adjust the Makefile to
remove the assumption that only PowerPC uses the emulator.

Signed-off-by: Simon Glass <sjg@chromium.org>