Cache and retrieve syscall arguments when needed.

Cache and retrieve syscall arguments when needed.
* runtime/task_finder.c (__stp_utrace_task_finder_target_syscall_entry):
  New function that caches syscall arguments for mmap(), munmap(), and
  mprotect() for use in __stp_utrace_task_finder_target_syscall_exit().
  (__stp_utrace_task_finder_target_syscall_exit): Uses cached syscall
  argument info when making callbacks.
  (stap_start_task_finder): Initializes map subsytem for storing syscall
  argument data.
* runtime/task_finder_map.c: New file containing functions to save and
  retrieve syscall arguments.