[release-branch.go1.4] runtime: fix SIGPROF change

CL 2789 backported a change that required a barrage of followup CLs.
This CL backports all the followup CLs together.

There are manual edits to os_plan9.go and syscall_windows.go to take
the place of edits to defs_windows_{amd64,386}.go and os2_plan9.go
in the original. Those files do not exist in the release branch, but the
definition being added must go somewhere.

Original change descriptions below.

---

runtime/cgo: initialize our pthread_create wrapper earlier on openbsd

This is a genuine bug exposed by our test for issue 9456: our wrapper
for pthread_create is not initialized until we initialize cgo itself,
but it is possible that a static constructor could call pthread_create,
and in that case, it will be calling a nil function pointer.

Fix that by also initializing the sys_pthread_create function pointer
inside our pthread_create wrapper function, and use a pthread_once to
make sure it is only initialized once.

Fix build for openbsd.

Change-Id: Ica4da2c21fcaec186fdd3379128ef46f0e767ed7
Reviewed-on: https://go-review.googlesource.com/2232


Reviewed-by: David Crawshaw <crawshaw@golang.org>
(cherry picked from commit 77cd6197)

---

runtime: provide a dummy value of _SIGPROF on plan9 and windows

Fixes build on plan9 and windows.

Change-Id: Ic9b02c641ab84e4f6d8149de71b9eb495e3343b2
Reviewed-on: https://go-review.googlesource.com/2233


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
(cherry picked from commit 1f282385)

---

runtime/cgo: remove unused variable

I missed this one in golang.org/cl/2232 and only tested the patch
on openbsd/amd64.

Change-Id: I4ff437ae0bfc61c989896c01904b6d33f9bdf0ec
Reviewed-on: https://go-review.googlesource.com/2234


Reviewed-by: Minux Ma <minux@golang.org>
(cherry picked from commit 0b2a74e8)

---

runtime: skip TestCgoExternalThreadSIGPROF on OS X 10.6

The test program requires static constructor, which in turn needs
external linking to work, but external linking never works on 10.6.

This should fix the darwin-{386,amd64} builders.

Change-Id: I714fdd3e35f9a7e5f5659cf26367feec9412444f
Reviewed-on: https://go-review.googlesource.com/2235


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 2cbe27a2)

---

runtime: fix TestCgoExternalThreadSIGPROF again

Shell out to `uname -r` this time, so that the test will compile
even if the platform doesn't have syscall.Sysctl.

Change-Id: I3a19ab5d820bdb94586a97f4507b3837d7040525
Reviewed-on: https://go-review.googlesource.com/2271


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 865e5e98)

---

runtime: remove unnecessary GOOS switch

Change-Id: I8f518e273c02110042b08f7c50c3d38a648c8b6e
Reviewed-on: https://go-review.googlesource.com/2281


Reviewed-by: Minux Ma <minux@golang.org>
(cherry picked from commit 1ebfb082)

---

Change-Id: Ifee9667ca90eda2b074817c319b1b7c66d4f741d
Reviewed-on: https://go-review.googlesource.com/2805


Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>