cmd/gc: better error messages for C-style if statements.
Given
        if (i == 0)
                x++
The old message was
        x.go:6: syntax error: unexpected semicolon or newline before {
Now we see
        x.go:6: syntax error: missing { after if clause

Fixes issue 5687

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/12822045
cmd/gc: don't attempt to generate wrappers for blank interface methods

Fixes issue 5691.

R=golang-dev, bradfitz, daniel.morsing, rsc
CC=golang-dev
https://codereview.appspot.com/10255047

Committer: Rob Pike <r@golang.org>