isNoop is not thread safe

[wangbj]

https://github.com/upenn-acg/detTrace/blob/13a37abf2b0babfe6d85f71defeb45f62adf41f6/src/rnr_loader.cpp#L90

Each threads can do syscalls in parallel, isNoop could have been overwritten by another thread if current thread is de-scheduled. It's better keep the rnr prehook/posthook simpler, by moving those noop checking into the actual rnr tool if they're really needed.