OpenBSD 6.0 (due out November 2016) will support PT_TLS, which will
allow for the OpenBSD cgo pthread_create() workaround to be removed.

However, in order for Go to continue working on supported OpenBSD
releases (the current release and the previous release - 5.9 and 6.0,
once 6.0 is released), we cannot enable PT_TLS immediately. Instead,
adjust the existing code so that it works with the previous TCB
allocation and the new TIB allocation. This allows the same Go
runtime to work on 5.8, 5.9 and later 6.0.

Once OpenBSD 5.9 is no longer supported (May 2017, when 6.1 is
released), PT_TLS can be enabled and the additional cgo runtime
code removed.

Change-Id: I3eed5ec593d80eea78c6656cb12557004b2c0c9a
Reviewed-on: https://go-review.googlesource.com/23197Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>