stacks: call check_irqs() in run_thread()

The comment above the yield() function suggests that yield()
allows interrupts for a short time. But yield() only briefly
enables interrupts if seabios was built without CONFIG_THREADS
or if yield() is called from the main thread. In order to
guarantee that the interrupts were enabled once before yield()
returns in a background thread, the main thread must call
check_irqs() before or after every thread switch. The function
run_thread() also switches threads, but the call to check_irqs()
was forgotten. Add the missing check_irqs() call.

This fixes PS/2 keyboard initialization failures.

The code in src/hw/ps2port.c relies on yield() to briefly enable
interrupts. There is a comment above the yield() function in
__ps2_command(), where the author left a remark why the call to
yield() is actually needed.

Here is one of the call sequences leading to a PS/2 keyboard
initialization failure.

ps2_keyboard_setup()
  |
  ret = i8042_command(I8042_CMD_CTL_TEST, param);
  # This command will register an interrupt if the PS/2 device
  # controller raises interrupts for replies to a controller
  # command.
  |
  ret = ps2_kbd_command(ATKBD_CMD_RESET_BAT, param);
    |
    ps2_command(0, command, param);
      |
      ret = __ps2_command(aux, command, param);
        |
        // Flush any interrupts already pending.
        yield();
        # yield() doesn't flush interrupts if the main thread
        # hasn't reached wait_threads().
        |
        ret = ps2_sendbyte(aux, command, 1000);
        # Reset the PS/2 keyboard controller and wait for
        # PS2_RET_ACK.
        |
        ret = ps2_recvbyte(aux, 0, 4000);
          |
          for (;;) {
            |
            status = inb(PORT_PS2_STATUS);
            # I8042_STR_OBF isn't set because the keyboard self
            # test reply is still on wire.
            |
            yield();
            # After a few yield()s the keyboard interrupt fires
            # and clears the I8042_STR_OBF status bit. If the
            # keyboard self test reply arrives before the
            # interrupt fires the keyboard reply is lost and
            # ps2_recvbyte() returns after the timeout.
          }

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>

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