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.

third_party/lua/ltests.c

@@ -1262,7 +1262,7 @@ static int panicback (lua_State *L) {
   b = (struct Aux *)lua_touserdata(L, -1);
   lua_pop(L, 1);  /* remove 'Aux' struct */
   runC(b->L, L, b->paniccode);  /* run optional panic code */
-  _gclongjmp(b->jb, 1);
+  gclongjmp(b->jb, 1);
   return 1;  /* to avoid warnings */
 }