cmd/link: should pass -Wl,-z,relro when linking with -buildmode=pie

[mwhudson]

By my reading of the code, we currently do all the prep in the linker to work with -zrelro when linking with -buildmode=pie, but we don't actually pass it to the linker.

@crawshaw ?

[crawshaw]

That's what it looks like. But then, how does anything work?

[ianlancetaylor]

Many GNU/Linux distros pass -z relro by default.

[gopherbot]

CL https://golang.org/cl/22687 mentions this issue.