runtime: add type cast for non-split-stack calls to scanstackblock
runtime: add type cast for non-split-stack calls to scanstackblock

Change-Id: I4defa593595429651bfe70b10a82d70ea55fa0e2
Reviewed-on: https://go-review.googlesource.com/c/162884
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend