runtime: add netpollBreak

The new netpollBreak function can be used to interrupt a blocking netpoll.
This function is not currently used; it will be used by later CLs.

Updates #27707

Change-Id: I5cb936609ba13c3c127ea1368a49194fc58c9f4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/171824
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
diff --git a/src/runtime/defs_freebsd_arm.go b/src/runtime/defs_freebsd_arm.go
index cf7ca69..3307c8b 100644
--- a/src/runtime/defs_freebsd_arm.go
+++ b/src/runtime/defs_freebsd_arm.go
@@ -15,6 +15,7 @@
 const (
 	_EINTR  = 0x4
 	_EFAULT = 0xe
+	_EAGAIN = 0x23
 	_ENOSYS = 0x4e
 
 	_O_NONBLOCK = 0x4