teensy3: usb_dev: Mark _reboot_Teensyduino_() unreachable

When including "core_pins.h":

    teensy3/usb_dev.c: In function '_reboot_Teensyduino_':
    teensy3/usb_dev.c:899:1: warning: 'noreturn' function does return

Fix this by adding a call to __builtin_unreachable().

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

teensy3/usb_dev.c

 {
 	// TODO: initialize R0 with a code....
 	__asm__ volatile("bkpt");
+	__builtin_unreachable();
 }