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.

libc/thread/tls.h

@@ -65,7 +65,7 @@ void __set_tls(struct CosmoTib *);
  *
  * This can't be used in privileged functions.
  */
-__funline pureconst struct CosmoTib *__get_tls(void) {
+forceinline pureconst struct CosmoTib *__get_tls(void) {
 #ifdef __chibicc__
   return 0;
 #elif __x86_64__