runtime: report correct fcntl syscall error in checkCloseonexec

runtime: report correct fcntl syscall error in checkCloseonexec

runtime.fcntl returns the error value as a negative value, so it needs
to be inverted before being converted to syscall.Errno.

Change-Id: I43cd0b035150424ac59e623b17a9396c7d62c186
Reviewed-on: https://go-review.googlesource.com/c/go/+/495675
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>