cmd/pprof: install as go tool pprof

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/168320043
cmd/dist: remove old misc/pprof

LGTM=dave, bradfitz, r, alex.brainman
R=r, dave, bradfitz, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/167350043
runtime/cgo: add +build tags to files named for $GOOS

These are being built into the runtime/cgo for every
operating system. It doesn't seem to matter, but
restore the Go 1.3 behavior anyway.

LGTM=r
R=r, dave
CC=golang-codereviews
https://codereview.appspot.com/171290043
runtime: fix sudog leak in syncsemrelease

Manifested as increased memory usage in a Google production system.

Not an unbounded leak, but can significantly increase the number
of sudogs allocated between garbage collections.

I checked all the other calls to acquireSudog.
This is the only one that was missing a releaseSudog.

LGTM=r, dneil
R=dneil, r
CC=golang-codereviews
https://codereview.appspot.com/169260043
cmd/internal/objfile: minor edits

Follow-up in response to comments on
TBR'ed CL 171260043.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/172080043