cmd/cc, runtime: disallow structs without tags

Structs without tags have no unique name to use in the
Go definitions generated from the C types.
This caused issue 8812, fixed by CL 149260043.
Avoid future problems by requiring struct tags.

Update runtime as needed.
(There is no other C code in the tree.)

LGTM=bradfitz, iant
R=golang-codereviews, bradfitz, dave, iant
CC=golang-codereviews, khr, r
https://codereview.appspot.com/150360043