Add optional TAU instrumentation to XGC

[germasch]

Two non-invasive options for XGC:

1. Depending on a config option, don’t link with camtimers but instead with a thin wrapper that provides GPTL{start, stop} which forward the calls to perfstubs.
2. Extend the camtimers implementation to, if configured accordingly, call perfstubs from within camtimers.

(2) has potential drawbacks in that it means that the camtimers included within xgc will diverge from the official version (and XGC doesn’t always use the camtimers that are packaged with it, I believe it typically uses preinstalled versions on commonly used machines). It also may mean that the backtraces always point into camtimers rather than the original routines — which may not matter. On the other hand (2) has the advantage that camtimers will coexist with tau in the same run, which may or may not be important.

[germasch]

SVM: I like option (1). These two options are the same work for Kevin, I think