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/zlib/zlib.h

@@ -171,7 +171,7 @@ typedef z_stream *z_streamp;
  */
 typedef struct gz_header_s {
   int text;       /* true if compressed data believed to be text */
-  uLong time;     /* modification time */
+  uLong time_;    /* modification time */
   int xflags;     /* extra flags (not used when writing a gzip file) */
   int os;         /* operating system */
   Bytef *extra;   /* pointer to extra field or Z_NULL if none */