go/gcimporter15: update to match latest changes to cmd/compiler exporter

Backport of https://go-review.googlesource.com/27814 changes to
bexport.go.

With this, the gcimporter15 is again up-to-date and in sync with both
the compiler's export code, and the compiler's and std lib gcimporter's
import code.

Change-Id: I8e229660eb78ddc1506b7b96a89c81ff083e0412
Reviewed-on: https://go-review.googlesource.com/27914
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
1 file changed