vgabios: Support allocating an extra stack for vgabios calls and default on.

Add code to allocate an extra stack for the main vgabios int 0x10
entry point.  The allocation is done via the PMM spec and uses a PCI
v3 permanent low memory region request.  This request will work with
SeaBIOS - it is unknown how many other main BIOS implementations
support this PMM call.

The extra stack is useful for old DOS programs that call the VGABIOS
and expect it to work with very small amounts of stack space.

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