Just open-code the fcntl syscall instead of relying on the obscurity of
go:linkname.

Change-Id: I3e4ec9db6539e016f56667d7b8b87aa37671d0e7
Reviewed-on: https://go-review.googlesource.com/130736
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>