cpu,gbdb: implement better breakpoints

Previously, gbdb had to check several signals, including the current
PC, to determine when do_run() should stop. This code was hotpath and
unecessarily slow.

Instead, leverage a n undefined instruction (0xd3) as a breakpoint instruction.
When the CPU emulation encounters this instruction, it will call a callback
which is implemented by gbdb. This can set a simple flag which is less expensive to
query.

Finally, both the signal handler and the breakpoint callback set specific
"paused" flags and a generic "pause" flag. Now, do_run() can simply check
the generic "paused" flag, then use the specific flags to determine the
stop reason.

This change increased performance by ~10% on Raspberry Pi.

src/gbemu/cpu.h

@@ -94,8 +94,11 @@ struct lr35902_state {
 
 	int stall_cycles;
 	int halted;
+
 	lr35902_mem_read_fn mem_read;
 	lr35902_mem_write_fn mem_write;
+	void (*undef_d3)(struct lr35902_state *reg);
+
 	lr35902_interrupt_state int_state;
 	struct lr35902_event events[LR35902_MAX_EVENTS];
 
@@ -108,9 +111,15 @@ struct lr35902_state {
 	} metrics;
 };
 
+struct lr35902_ops {
+  lr35902_mem_read_fn mem_read;
+  lr35902_mem_write_fn mem_write;
+
+  void (*undef_d3)(struct lr35902_state *reg);
+};
+
 void lr35902_init(struct lr35902_state *cpu,
-                  lr35902_mem_read_fn mem_read,
-                  lr35902_mem_write_fn mem_write);
+		  const struct lr35902_ops *ops);
 
 uint16_t lr35902_get_reg_16(const struct lr35902_state *cpu,
                             lr35902_regs_16 reg);