runtime: Change size of map iter offset so 32-bit version compiles cleanly.
runtime: Change size of map iter offset so 32-bit version compiles cleanly.

R=golang-codereviews, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/52310043

tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go