rewrite __synccall in terms of global thread list

the __synccall mechanism provides stop-the-world synchronous execution
of a callback in all threads of the process. it is used to implement
multi-threaded setuid/setgid operations, since Linux lacks them at the
kernel level, and for some other less-critical purposes.

this change eliminates dependency on /proc/self/task to determine the
set of live threads, which in addition to being an unwanted dependency
and a potential point of resource-exhaustion failure, turned out to be
inaccurate. test cases provided by Alexey Izbyshev showed that it
could fail to reflect newly created threads. due to how the
presignaling phase worked, this usually yielded a deadlock if hit, but
in the worst case it could also result in threads being silently
missed (allowed to continue running without executing the callback).

src/internal/pthread_impl.h

@@ -139,7 +139,6 @@ hidden void __pthread_tsd_run_dtors();
 hidden void __pthread_key_delete_synccall(void (*)(void *), void *);
 hidden int __pthread_key_delete_impl(pthread_key_t);
 
-extern hidden volatile int __block_new_threads;
 extern hidden volatile size_t __pthread_tsd_size;
 extern hidden void *__pthread_tsd_main[];
 extern hidden volatile int __aio_fut;