[release-branch.go1.4] runtime: fix nacl build, hashmap overflow field offset was...

[release-branch.go1.4] runtime: fix nacl build, hashmap overflow field offset was incorrect.

Change-Id: Ieb305b2a4d4ef28d70a8b8ece703f495c5af0529
Reviewed-on: https://go-review.googlesource.com/2051
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit c6669e7af58cb9406226c038b1cadef6e754dc74)
Reviewed-on: https://go-review.googlesource.com/2820
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>