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/calls/tmpfd.c

@@ -98,8 +98,7 @@ int tmpfd(void) {
                    0600)) != -1) {
       if (!IsWindows()) {
         if (unlink(path)) {
-          asm("hlt");
-          unreachable;
+          notpossible;
         }
       }
       return fd;