Add the AMI Excalibur, a VLB OPTi 596/597 machine.

Also adds emulation of the OPTi 5x7 chipset, and introduces a clock divider for VLB on 64-bit bus systems.

src/include/86box/machine.h

@@ -253,6 +253,8 @@ extern const device_t 	*at_cpqiii_get_device(void);
 #endif
 
 /* m_at_socket4_5.c */
+extern int  machine_at_excalibur_init(const machine_t *);
+
 extern int	machine_at_batman_init(const machine_t *);
 extern int	machine_at_ambradp60_init(const machine_t *);
 #if defined(DEV_BRANCH) && defined(USE_VPP60)