add time64 redirect for, and redirecting implementation of, dlsym

if symbols are being redirected to provide the new time64 ABI, dlsym
must perform matching redirections; otherwise, it would poke a hole in
the magic and return pointers to functions that are not safe to call
from a caller using time64 types.

rather than duplicating a table of redirections, use the time64
symbols present in libc's symbol table to derive the decision for
whether a particular symbol needs to be redirected.

include/dlfcn.h

@@ -35,6 +35,10 @@ int dladdr(const void *, Dl_info *);
 int dlinfo(void *, int, void *);
 #endif
 
+#if _REDIR_TIME64
+__REDIR(dlsym, __dlsym_time64);
+#endif
+
 #ifdef __cplusplus
 }
 #endif