kernel: use uint64 instead of int64 to avoid narrowing conversion warnings.

* gdb_register is used only in arch_debug.cpp

src/system/kernel/arch/x86/arch_debug.cpp

@@ -1252,7 +1252,7 @@ arch_get_debug_variable(const char* variableName, uint64* value)
 
 struct gdb_register {
 	int32 type;
-	int64 value;
+	uint64 value;
 };