runtime: correct setNonblock on linux/arm64

The current code uses EOR (exclusive OR), which will result in the O_NONBLOCK
flag being toggled rather than being set. Other implementations use OR, hence
this is likely a bug.

Change-Id: I5dafa9c572452070bd37789c8a731ad6d04a86cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/212766


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>