fix compile regression in exit on archs without SYS_pause

commit 8cca79a72cccbdb54726125d690d7d0095fc2409 added use of SYS_pause
to exit() without accounting for newer archs omitting the syscall.

use the newly-added __sys_pause abstraction instead, which uses
SYS_ppoll when SYS_pause is missing.

src/exit/exit.c

@@ -38,7 +38,7 @@ _Noreturn void exit(int code)
 	int tid =  __pthread_self()->tid;
 	int prev = a_cas(exit_lock, 0, tid);
 	if (prev == tid) a_crash();
-	else if (prev) for (;;) __syscall(SYS_pause);
+	else if (prev) for (;;) __sys_pause();
 
 	__funcs_on_exit();
 	__libc_exit_fini();