cmd/go: add GOOS and GOARCH to generate
Fixes test failure in build, probably a good idea anyway.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/131210043
cmd/gc, runtime: treat slices and strings like pointers in garbage collection

Before, a slice with cap=0 or a string with len=0 might have its
base pointer pointing beyond the actual slice/string data into
the next block. The collector had to ignore slices and strings with
cap=0 in order to avoid misinterpreting the base pointer.

Now, a slice with cap=0 or a string with len=0 still has a base
pointer pointing into the actual slice/string data, no matter what.
The collector can now always scan the pointer, which means
strings and slices are no longer special.

Fixes issue 8404.

LGTM=khr, josharian
R=josharian, khr, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/112570044
crypto/cipher: add CFB test vectors.

Fixes issue 8576.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/136720043
net/http: populate Request.Close in ReadRequest

Fixes Issue 8261

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/126620043