Add notpossible keyword

This is the same as `unreachable` except it always traps violations,
even if we're not running in MODE=dbg. This is useful for impossible
conditions relating to system calls. It avoids terrifying bugs where
control falls through to an unrelated function.

libc/runtime/abort.c

@@ -40,6 +40,5 @@ wontreturn void abort(void) {
   raise(SIGABRT);
   signal(SIGABRT, SIG_DFL);
   raise(SIGABRT);
-  asm("hlt");
-  unreachable;
+  notpossible;
 }