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

third_party/chibicc/test/arith_test.c

@@ -1,6 +1,7 @@
 #include "third_party/chibicc/test/test.h"
 
 int main() {
+
   ASSERT(0, 0);
   ASSERT(42, 42);
   ASSERT(21, 5 + 20 - 4);