runtime: scan register backing store on ia64

On ia64, a separate stack is used for saving/restoring register frames,
occupying the other end of the stack mapping. This must also be scanned
for pointers into the heap.

Change-Id: I84bc1f9f0aff25ed7a2302777764ebaf89e613be
Reviewed-on: https://go-review.googlesource.com/85276
Reviewed-by: Ian Lance Taylor <iant@golang.org>
4 files changed