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/execve-nt.greg.c

@@ -121,5 +121,5 @@ textwindows int sys_execve_nt(const char *program, char *const argv[],
   } while (dwExitCode == kNtStillActive);
   __imp_CloseHandle(procinfo.hProcess);
   __imp_ExitProcess(dwExitCode);
-  unreachable;
+  notpossible;
 }