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/python/Modules/_decimal/libmpdec/transpose.c

@@ -28,7 +28,7 @@
 │ EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.                           │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "third_party/python/Modules/_decimal/libmpdec/transpose.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/mem/mem.h"
 #include "third_party/python/Modules/_decimal/libmpdec/bits.h"
 #include "third_party/python/Modules/_decimal/libmpdec/constants.h"

third_party/python/Modules/_threadmodule.c

@@ -27,7 +27,7 @@
 #include "libc/errno.h"
 #include "libc/log/log.h"
 #include "libc/mem/alg.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/nt/createfile.h"
 #include "libc/nt/dll.h"
 #include "libc/nt/enum/creationdisposition.h"

third_party/python/Modules/getpath.c

@@ -11,7 +11,7 @@
 #include "libc/intrin/cmpxchg.h"
 #include "libc/log/log.h"
 #include "libc/mem/alloca.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/mem/mem.h"
 #include "libc/runtime/runtime.h"
 #include "libc/str/str.h"

third_party/python/Modules/posixmodule.c

@@ -29,7 +29,7 @@
 #include "libc/errno.h"
 #include "libc/log/log.h"
 #include "libc/mem/alg.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/nt/createfile.h"
 #include "libc/nt/dll.h"
 #include "libc/nt/enum/creationdisposition.h"

third_party/python/Modules/selectmodule.c

@@ -7,7 +7,7 @@
 #include "libc/calls/calls.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/mem/mem.h"
 #include "libc/nt/efi.h"
 #include "libc/sock/epoll.h"

third_party/python/Modules/tlsmodule.c

@@ -20,7 +20,7 @@
 #include "libc/calls/calls.h"
 #include "libc/errno.h"
 #include "libc/macros.internal.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/str/str.h"
 #include "net/https/https.h"
 #include "third_party/mbedtls/ctr_drbg.h"