cmd/gc: fix Offsetof computation.

The offset of an embedded field s.X must be relative to s
and not to the implicit s.Field of which X is a direct field.
Moreover, no indirections may happen on the path.

Fixes issue 4909.

R=nigeltao, ality, daniel.morsing, iant, gri, r
CC=golang-dev
https://codereview.appspot.com/8287043