Only include unwaddr in context for ia64. Not used on any other arch.

Only include unwaddr in context for ia64. Not used on any other arch.

* translate.cxx (c_unparser::emit_common_header): Don't add unwaddr field
  to context struct except for ia64.
* tapsets.cxx (common_probe_entryfn_prologue): Only initialized unwaddr
  on ia64.
  (dwarf_derived_probe::emit_probe_local_init): Only call bspcache on ia64.
* runtime/regs-ia64.c: Don't define (empty) bspcache for other arches.