fix a few bugs in scheduling and synchronization, separate OVMF_VARS for debug

code/hal/hal_timer.c

@@ -23,7 +23,7 @@ static volatile uint32_t* gLapic = NULL;
 #define LAPIC_REG(off) (*(volatile uint32_t *)((uint8_t *)gLapic + (off)))
 
 static volatile uint64_t gTicks = 0;
-void (*gTimerCallback)(void) = NULL;
+void (* volatile gTimerCallback)(void) = NULL;
 
 static inline void outb(uint16_t port, uint8_t val) { __outbyte(port, val); }
 static inline uint8_t inb(uint16_t port)            { return __inbyte(port); }