cmd/go: build non-runnable examples in xtests

Include these files in the build,
even though they don't get executed.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/108180043
runtime: fix nacl amd64p32 flakiness

newproc takes two extra pointers, not two extra registers.
On amd64p32 (nacl) they are different.

We diagnosed this before the 1.3 cut but the tree was frozen.
I believe this is causing the random problems on the builder.

Fixes issue 8199.

TBR=r
CC=golang-codereviews
https://codereview.appspot.com/102710043
runtime: make garbage collector faster by deleting code again
Remove GC bitmap backward scanning.
This was already done once in https://codereview.appspot.com/5530074/
Still makes GC a bit faster.
On the garbage benchmark, before:
        gc-pause-one=237345195
        gc-pause-total=4746903
        cputime=32427775
        time=32458208
after:
        gc-pause-one=235484019
        gc-pause-total=4709680
        cputime=31861965
        time=31877772
Also prepares mgc0.c for future changes.

R=golang-codereviews, khr, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/105380043
net/http: add TLS benchmark

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/110080045
flag: add a little more doc comment to Duration.

The only text that describes the accepted format is in the package doc,
which is far away from these functions. The other flag types don't need
this explicitness because they are more obvious.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/101550043
crypto/cipher: Fix typo in example comment

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/101510047

Committer: Brad Fitzpatrick <bradfitz@golang.org>
strings: Replacer is safe for concurrent use

LGTM=r
R=golang-codereviews, bradfitz, r
CC=golang-codereviews
https://codereview.appspot.com/109220044

Committer: Rob Pike <r@golang.org>
runtime: fix GC bitmap corruption
Fixes issue 8299.

R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/103640044
test/fixedbugs: fix typo in comment

Fix copy paste error pointed out by rsc, https://codereview.appspot.com/107290043/diff/60001/test/fixedbugs/issue8074.go#newcode7

LGTM=ruiu, r
R=golang-codereviews, ruiu, r
CC=golang-codereviews
https://codereview.appspot.com/106210047