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/log/attachdebugger.c

@@ -108,7 +108,7 @@ relegated int(AttachDebugger)(intptr_t continuetoaddr) {
                      "c",      NULL,
                  },
                  environ);
-    __builtin_trap();
+    abort();
   }
   return pid;
 }