Clean up more code

- Found some bugs in LLVM compiler-rt library
- The useless LIBC_STUBS package is now deleted
- Improve the overflow checking story even further
- Get chibicc tests working in MODE=dbg mode again
- The libc/isystem/ headers now have correctly named guards

libc/calls/oldbench.c

@@ -31,6 +31,8 @@
 #include "libc/thread/tls.h"
 #include "libc/time/time.h"
 
+// TODO(jart): DELETE
+
 static struct Now {
   bool once;
   uint64_t k0;