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/stdio/getentropy_test.c

@@ -64,7 +64,7 @@ TEST(getentropy, test) {
   double e, w = 7.7;
   struct sigaction sa;
   int i, k, m, n = 999;
-  char *buf = _gc(calloc(1, n));
+  char *buf = gc(calloc(1, n));
   sa.sa_flags = 0;
   sa.sa_handler = OnSig;
   sigemptyset(&sa.sa_mask);