cmd/cgo: use explicit flag to emit dynamic linker path

Using -import_runtime_cgo would have worked great except
that it doesn't get passed to the second invocation of cgo,
and that's the one that writes the relevant file.

Fixes ARM build on systems with a different dynamic linker
than the one 5l assumes (like Gentoo).

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7432048