PR10601 part 1: i386 and x86-64 regset for dwarf fetch/store_register()s

PR10601 part 1: i386 and x86-64 regset for dwarf fetch/store_register()s

* runtime/loc2c-runtime.h (fetch_register, store_register):
  forked into k_ (kernel) and u_ (user) varieties.  Implement
  i386 and x86_64 in terms of regset.h; fall back to k_* for
  other architectures.
* tapsets.cxx: (*::visit_target_symbol): Emit macros to map
  loc2c's fetch/store_register to loc2c-runtime's k_ or u_ as
  appopriate.