Extra SIGTRAP when breakpoint + watchpoint occur on same instruction
Description of problem:
If a breakpoint and watchpoint occur on the same instruction in TCG, gdb receives a breakpoint notification, a watchpoint notification, and then a SIGTRAP not corresponding to any set breakpoint/watchpoint.
Steps to reproduce:
Start QEMU via:

```
./qemu-system-i386 -display none -accel tcg -kernel kernel.elf -s -S
```

Here's the gdb session:

```
(gdb) file kernel.elf
Reading symbols from kernel.elf...done.
(gdb) tar rem :1234
Remote debugging using :1234
0x0000fff0 in ?? ()
(gdb) b _start
Breakpoint 1 at 0x10000c: file kernel.s, line 17.
(gdb) c
Continuing.

Breakpoint 1, _start () at kernel.s:17
17          mov eax, 3
(gdb) b bp
Breakpoint 2 at 0x100011: file kernel.s, line 20.
(gdb) watch *(int*)&value
Hardware watchpoint 3: *(int*)&value
(gdb) c
Continuing.

Breakpoint 2, bp () at kernel.s:20
20          mov dword ptr value, eax
(gdb) c
Continuing.

Hardware watchpoint 3: *(int*)&value

Old value = 0
New value = 3
done () at kernel.s:23
23          jmp done
(gdb) c
Continuing.

Program received signal SIGTRAP, Trace/breakpoint trap.
done () at kernel.s:23
23          jmp done
```
Additional information:
This patch fixes it by disabling the extra debug interrupt if the CPU is already singlestepping, but I'm not certain it's the 'correct' fix?

```patch
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -894,7 +894,9 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
          * trigger after the current instruction.
          */
         qemu_mutex_lock_iothread();
-        cpu_interrupt(cpu, CPU_INTERRUPT_DEBUG);
+        if ((cpu->singlestep_enabled & SSTEP_NOIRQ) == 0) {
+            cpu_interrupt(cpu, CPU_INTERRUPT_DEBUG);
+        }
         qemu_mutex_unlock_iothread();
         return;
     }

```