Fixed PR6791 by adding ia64 dwarfless register access.

Fixed PR6791 by adding ia64 dwarfless register access.

* tapset/ia64/registers.stp: Add dwarfless register access.
* runtime/regs.c (__stp_sign_extend32): Ensure it is present for ia64
  compiles.
* tapset/linux/nd_syscalls2.stp: Add ia64 support.
* testsuite/systemtap.syscall/nd_syscall.exp: Remove kfail for ia64.
* testsuite/buildok/nd_syscalls-detailed.stp: Make accept4 and fork tests
  optional.
* testsuite/systemtap.examples/process/strace.tcl: Removed file that
  kfailed ia64 for lack of dwarfless register access.
* testsuite/systemtap.examples/process/thread-business.tcl: Ditto.