fix windows/amd64 build with newest mingw-w64
fix windows/amd64 build with newest mingw-w64

R=alex.brainman, golang-dev
CC=golang-dev
http://codereview.appspot.com/4968048

Committer: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>

apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go