linux-user/ia64: workaround ia64 strangenesses

linux-user/ia64: workaround ia64 strangenesses

ia64 has some strangenesses that need to be workaround:
- it has a __clone2() syscall instead of the using clone() one, with
  different arguments, and which is not declared in the usual headers.
- ucontext.uc_sigmask is declared with type long int, while it is
  actually of type sigset_t.
- uc_mcontext, uc_sigmask, uc_stack, uc_link are declared using #define,
  which clashes with the target_ucontext fields. Change their names to
  tuc_*, as already done for some target architectures.