Fix PR16636 by separating out preadv()/pwritev() testing.

Fix PR16636 by separating out preadv()/pwritev() testing.

* tapset/linux/syscalls2.stp (syscall.preadv): The 'fd' and 'count'
  variables are 32-bits on the user side. Truncate them.
  (syscall.pwritev): Ditto.
  (syscall.readv): Ditto.
  (syscall.writev): Ditto.
* testsuite/systemtap.syscall/readwrite.c: Move readv(), writev(),
  preadv(), pwritev() testing to their own files.
* testsuite/systemtap.syscall/preadv.c: New test.
* testsuite/systemtap.syscall/pwritev.c: Ditto.
* testsuite/systemtap.syscall/readv.c: Ditto.
* testsuite/systemtap.syscall/writev.c: Ditto.