cmd/gc: address &x.f is not resolved at link time

[randall77]

If you do
var y=&x

Then global y points to global x, and that reference is computed at link time.

If you do (where f is a field of x)
var y = &x.f

This reference is not computed at link time. Instead, it is set up dynamically by init(). This means that other code during init can see y as nil.

See #9354 for why this matters.

[ianlancetaylor]

I don't understand how other code can see y as nil. That sounds like a bug in the init dependency ordering.

[minux]

The problem only happens in runtime where runtime.init is delayed.

[randall77]

In any case, it is worth fixing regardless. No need to compute this address at runtime when it is easily done at link time.

[minux]

Sure. https://gist.github.com/minux/69994348b4f06b1c324b
fixes the underlying problem for 6g.

If you like the approach, I can finish target-specific code for
other ports, and mail it out.

We also need a test, which is harder (other than perhaps verifying
the output of gc)

[randall77]

I have a competing CL to fix this bug. It's a bit more general, and it has a shiny test.
https://go-review.googlesource.com/1691