harden thread start with failed scheduling against broken __clone

commit 8a544ee3a2 introduced a
dependency of the failure path for explicit scheduling at thread
creation on __clone's handling of the start function returning, which
should result in SYS_exit.

as noted in commit 05870abeaa, the arm
version of __clone was broken in this case. in the past, the mips
version was also broken; it was fixed in commit
8b2b61e000.

since this code path is pretty much entirely untested (previously only
reachable in applications that call the public clone() and return from
the start function) and consists of fragile per-arch asm, don't assume
it works, at least not until it's been thoroughly tested. instead make
the SYS_exit syscall from the start function's failure path.

src/thread/pthread_create.c

@@ -185,7 +185,7 @@ static int start(void *p)
 			__wait(&args->control, 0, 2, 1);
 		if (args->control) {
 			__syscall(SYS_set_tid_address, &args->control);
-			return 0;
+			for (;;) __syscall(SYS_exit, 0);
 		}
 	}
 	__syscall(SYS_rt_sigprocmask, SIG_SETMASK, &args->sig_mask, 0, _NSIG/8);