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/proc/execve-sysv.c

@@ -58,6 +58,7 @@ static bool IsApeFile(const char *path) {
   } else {
     bool res = false;
     BLOCK_SIGNALS;
+    BLOCK_CANCELATION;
     int fd;
     char buf[8];
     int flags = O_RDONLY | O_NOCTTY | O_NONBLOCK | O_CLOEXEC;
@@ -65,6 +66,7 @@ static bool IsApeFile(const char *path) {
       res = sys_pread(fd, buf, 8, 0, 0) == 8 && IsApeLoadable(buf);
       sys_close(fd);
     }
+    ALLOW_CANCELATION;
     ALLOW_SIGNALS;
     return res;
   }