Fixed PR16311 by adding/improving pwritev syscall support.

Fixed PR16311 by adding/improving pwritev syscall support.

* tapset/linux/syscalls2.stp: Improved syscall.pwritev probe support,
  fixed offset calculations.
* tapset/linux/nd_syscalls2.stp: Added 'nd_syscall.pwritev' and
  'nd_syscall.pwritev.return' probes.
* testsuite/buildok/syscalls2-detailed.stp: Added pwritev probe test.
* testsuite/buildok/nd_syscalls2-detailed.stp: Ditto.
* testsuite/systemtap.syscall/readwrite.c: Aded pwritev() test.