PR16716 partial fix: Fix types in '[nd_]syscall.eventfd'.

PR16716 partial fix: Fix types in '[nd_]syscall.eventfd'.

* tapset/linux/syscalls.stp (syscall.eventfd): Better handle eventfd()
  vs. eventfd2(). Add 'count' and 'flags_str' convenience variables. Fix
  types.
* tapset/linux/nd_syscalls.stp: Ditto.
* tapset/linux/aux_syscalls.stp (_eventfd2_flag_str): Updated and improved.
* runtime/linux/compat_unistd.h (__NR_compat_eventfd2): New define.
* testsuite/systemtap.syscall/eventfd.c: Add more tests.
* testsuite/buildok/syscalls-detailed.stp: Add tests for new convenience
  variables.
* testsuite/buildok/nd_syscalls-detailed.stp: Ditto.