testing: report test as failed if the test panics.

Fixes issue 5149.

R=golang-dev, dave, minux.ma
CC=golang-dev
https://codereview.appspot.com/8136043

Committer: Dave Cheney <dave@cheney.net>
cmd/go: prevent packages from being cleaned more than once

If a package was listed as a dependency from multiple places, it
could have been cleaned repeatedly.

R=golang-dev, dave, rsc, seed, bradfitz
CC=golang-dev, minux.ma
https://codereview.appspot.com/7482043

Committer: Brad Fitzpatrick <bradfitz@golang.org>
runtime: add concurrent map read test

Currently crashes, so disabled.

Update issue 5179

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/8222044
cmd/gc: do not reuse bool temporaries for composite equality.

Reusing it when multiple comparisons occurred in the same
function call led to bad overwriting.

Fixes issue 5162.

R=golang-dev, daniel.morsing
CC=golang-dev
https://codereview.appspot.com/8174047
cmd/ld: add -extld and -extldflags options

Permits specifying the linker to use, and trailing flags to
pass to that linker, when linking in external mode.  External
mode linking is used when building a package that uses cgo, as
described in the cgo docs.

Also document -linkmode and -tmpdir.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8225043