[IA64] access user RBS directly

Because the user RBS of a process is now completely stored in
user-mode when the process is ptrace-stopped, accesses to the
RBS should no longer augment any part of the kernel RBS.

This means we can get rid of most ia64_peek() and ia64_poke()
calls.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Signed-off-by: Tony Luck <tony.luck@intel.com>