runtime: prevent pointless jmp in amd64 and 386 memmove

6a and 8a rearrange memmove such that the fallthrough from move_1or2 to move_0 ends up being a JMP to a RET. Insert an explicit RET to prevent such silliness.

Do the same for memclr as prophylaxis.

benchmark                old ns/op     new ns/op     delta
BenchmarkMemmove1        4.59          4.13          -10.02%
BenchmarkMemmove2        4.58          4.13          -9.83%

LGTM=khr
R=golang-codereviews, dvyukov, minux, ruiu, bradfitz, khr
CC=golang-codereviews
https://codereview.appspot.com/120930043
test/run: go fmt

LGTM=josharian, r
R=golang-codereviews, josharian, r
CC=golang-codereviews
https://codereview.appspot.com/120160043
runtime: move constants from map header to map type

A good cleanup anyway, and it makes some room for an additional
field needed for issue 8412.

Update issue 8412

LGTM=iant
R=iant, khr
CC=golang-codereviews
https://codereview.appspot.com/112700043
cmd/go: use correct link flags if main package contains C++/ObjC files

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/117470043

Committer: Ian Lance Taylor <iant@golang.org>
net: fix Dial comment about IPv6 addresses

LGTM=r
R=golang-codereviews, r
CC=adg, golang-codereviews
https://codereview.appspot.com/118550043