Add a mode to use BIOS to enable A20 line

If USE_BIOS_FOR_A20 is defined, the BIOS is used to enable the A20 line.

"MS-DOS Player for Win32-x64" doesn't support the classical A20 control
via the keyboard controller. It only supports the BIOS call int 15h
ax=2401h to enable the A20 line.
Merge pull request #10 from k-takata/use-bios-for-a20

Add a mode to use BIOS to enable A20 line