cmd/gc: mark wrapper methods for unnamed types as DUPOK.

Unnamed types like structs with embedded fields can have methods.
These methods are generated on-the-fly by the compiler and
it may happen for identical types in different packages.
The linker must accept these multiple definitions.

Fixes issue 4590.

R=golang-dev, rsc
CC=golang-dev, remy
https://codereview.appspot.com/7030051
sync/atomic: document that users must deal with 64-bit alignment

Update issue 599.

R=dvyukov, iant, minux.ma
CC=golang-dev
https://codereview.appspot.com/7001056