Defines starting with `x' in mint

• XBIOS mint/locore.h:5
• XBIOS_MAX mint/xbios.c:25
• XBRA_MAGIC mint/xbra.h:10
• xconin mint/bios.c:61
• xconout mint/bios.c:63
• xconstat mint/bios.c:60
• xcostat mint/bios.c:62