cmd/8g: add descriptions for some missing instructions.

These instructions are emitted when GO386=387 or the target
i386 CPU does not have SSE2 capabilities.

Fixes issue 6215.

R=golang-dev, remyoudompheng
CC=golang-dev
https://codereview.appspot.com/12812045

Committer: R?my Oudompheng <oudomphe@phare.normalesup.org>
cmd/gc: make method names for function scoped types unique

Types in function scope can have methods on them if they embed another type, but we didn't make the name unique, meaning that 2 identically named types in different functions would conflict with eachother.

Fixes issue 6269.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13326045