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/tool/net/sqlite_test.c

@@ -122,7 +122,7 @@ TEST(sqlite, test) {
                                   ")"));
   ASSERT_EQ(SQLITE_OK, sqlite3_close(db));
   int i, n = 4;
-  pthread_t *t = _gc(malloc(sizeof(pthread_t) * n));
+  pthread_t *t = gc(malloc(sizeof(pthread_t) * n));
   for (i = 0; i < n; ++i) {
     ASSERT_EQ(0, pthread_create(t + i, 0, Worker, 0));
   }