go/gcimporter: update importer to match latest gc export data changes

Adjustments taken from https://golang.org/cl/22580.

Change-Id: Ic88137b410767bd17e3d6142cec2b5a112df56be
Reviewed-on: https://go-review.googlesource.com/22582
Reviewed-by: Alan Donovan <adonovan@google.com>
2 files changed