runtime: fix cgo callbacks on windows

Fixes issue 4955.

R=golang-dev, alex.brainman
CC=golang-dev
https://codereview.appspot.com/7563043
cmd/ld: host linking support for linux/amd64

Still to do: non-linux and non-amd64.
It may work on other ELF-based amd64 systems too, but untested.

"go test -ldflags -hostobj $GOROOT/misc/cgo/test" passes.

Much may yet change, but this seems a reasonable checkpoint.

R=iant
CC=golang-dev
https://codereview.appspot.com/7369057