Implement call32 mechanism using SMIs.

Add support for jumping into 32bit mode using a System Management Mode
(SMM) handler.  When available, this allows SeaBIOS to transition to
32bit mode even when called in vm86 mode.  It will also prevent the
clobbering of the segment registers.

Currently, the SMM mode is only supported in QEMU when running in TCG
mode.  Also, QEMU v2.1 (or later) is needed for it to work when in
vm86 mode.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Move a20 code from system.c and ps2port.h to x86.h

Although the a20 functionality was originally implemented in the ps2
controller, that is just a historical artifact.  It's a core feature
of modern x86 cpus and the code is better located in the x86.h header.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Backup and restore a20 on call32_sloppy()

Previously, the a20 line would always be enabled and left on after
call32_sloppy().  The setting should really be backed up and restored
on each call.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>