runtime: correct runtime structfield type to match reflect

The offset field in structfield has changed to offsetAnon, and now
requires a shift to get the actual offset value.

Fixes golang/go#23391

Change-Id: I663997eb4a6e73c6e8333e29c305df469db94954
Reviewed-on: https://go-review.googlesource.com/92275
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed