Bring back gc() function

Renaming gc() to _gc() was a mistake since the better thing to do is put
it behind the _COSMO_SOURCE macro. We need this change because I haven't
wanted to use my amazing garbage collector ever since we renamed it. You
now need to define _COSMO_SOURCE yourself when using amalgamation header
and cosmocc users need to pass the -mcosmo flag to get the gc() function

Some other issues relating to cancelation have been fixed along the way.
We're also now putting cosmocc in a folder named `.cosmocc` so it can be
more safely excluded by grep --exclude-dir=.cosmocc --exclude-dir=o etc.

test/libc/thread/pthread_cancel_test.c

@@ -232,7 +232,7 @@ void *CpuBoundWorker(void *arg) {
   wontleak1 = malloc(123);
   (void)wontleak1;
   pthread_cleanup_push(free, wontleak1);
-  wontleak2 = _gc(malloc(123));
+  wontleak2 = gc(malloc(123));
   (void)wontleak2;
   ASSERT_EQ(0, pthread_setspecific(key, (void *)31337));
   ASSERT_EQ(0, pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, 0));