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.

net/https/chaincertificate.c

@@ -17,7 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/log/log.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "net/https/https.h"
 
 bool ChainCertificate(mbedtls_x509_crt *cert, mbedtls_x509_crt *parent) {

net/https/finishcertificate.c

@@ -37,8 +37,8 @@ struct Cert FinishCertificate(struct Cert *ca, mbedtls_x509write_cert *wcert,
   if ((rc = mbedtls_pk_check_pair(&cert->pk, key))) {
     FATALF("generate key (grep -0x%04x)", -rc);
   }
-  LogCertificate(_gc(xasprintf("generated %s certificate",
-                               mbedtls_pk_get_name(&cert->pk))),
-                 cert);
+  LogCertificate(
+      gc(xasprintf("generated %s certificate", mbedtls_pk_get_name(&cert->pk))),
+      cert);
   return (struct Cert){cert, key};
 }