liblink: make GO_ARGS the default for functions beginning with ·

If there is a leading ·, assume there is a Go prototype and
attach the Go prototype information to the function.
If the function is not called from Go and does not need a
Go prototype, it can be made file-local instead (using name<>(SB)).

This fixes the current BSD build failures, by giving functions like
sync/atomic.StoreUint32 argument stack map information.

Fixes issue 8753.

LGTM=khr, iant
R=golang-codereviews, iant, khr, bradfitz
CC=golang-codereviews, r, rlh
https://codereview.appspot.com/142150043
reflect: add Type.Comparable

Like most of the Type methods, the definition of Comparable
is what the Go spec says it is.

Fixes issue 7911.

LGTM=gri
R=gri, r
CC=golang-codereviews
https://codereview.appspot.com/144020043
net/mail: allow us-ascii encoding

Fixes issue 6611.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/14990045