[debugcommands][crash] add a variant for cortex-m

Touching around address 0 doesn't cause a cortex-m device to crash, but
branching to an aligned address absolutely will.

lib/debugcommands/debugcommands.c

@@ -432,9 +432,14 @@ static int cmd_timeh(int argc, const console_cmd_args *argv) {
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Warray-bounds"
 static int cmd_crash(int argc, const console_cmd_args *argv) {
+#if ARCH_ARM && ARM_ONLY_THUMB
+    /* a branch directly to an aligned address should trigger a fault */
+    asm("bx %0":: "r"(0));
+#else
     /* should crash */
     volatile uint32_t *ptr = (void *)1;
     *ptr = 1;
+#endif
 
     /* if it didn't, panic the system */
     panic("crash");