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/calls/getrandom.c

@@ -88,12 +88,14 @@ static ssize_t GetDevUrandom(char *p, size_t n) {
   int fd;
   ssize_t rc;
   BLOCK_SIGNALS;
+  BLOCK_CANCELATION;
   fd = sys_openat(AT_FDCWD, "/dev/urandom", O_RDONLY | O_CLOEXEC, 0);
   if (fd != -1) {
     rc = sys_read(fd, p, n);
   } else {
     rc = -1;
   }
+  ALLOW_CANCELATION;
   ALLOW_SIGNALS;
   return rc;
 }