Add more capabilities to systemtap backtracing

Add more capabilities to systemtap backtracing

* runtime/sym.c: simplified logic in _stp_snprint_addr because it doesn't need to be that verbose,
  also added ability to have file names and line numbers print in backtraces
* runtime/sym.h: added new symbol flag to generate a more "fuller" backtrace (includes file names
  and line numbers)
* tapset/linux/context-unwind.stp: added new backtrace tapset function that also prints file names
* tapset/linux/ucontext-unwind.stp: added new backtrace tapset function that also prints file names
  and functions to allow backtraces using a user provided pc & sp.