unix: fix freebsd/arm build, fix freebsd/{386,arm} tests

Re-run mkall.sh on FreeBSD-11.1-RELEASE armv6 for Capsicum consts and
POSIX.1-2008 path relative *at() syscalls.

Add errors_freebsd_arm.go generated semi-automatically using:
https://gist.github.com/paulzhol/c6d54dc2baf2a6bd204b343a15459661
Similar to https://golang.org/cl/51570. In order to handle C enum based
constants that mkerrors.sh missed.

Use fix from https://golang.org/cl/53639 to run Capsicum tests only on amd64.

Fixes golang/go#21136

Change-Id: I9681b033531090e6b1c5e2446a442deb39cf3066
Reviewed-on: https://go-review.googlesource.com/53691
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_freebsd_test.go

@@ -14,6 +14,7 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"golang.org/x/sys/unix"
@@ -157,6 +158,9 @@ func init() {
 }
 
 func TestCapEnter(t *testing.T) {
+	if runtime.GOARCH != "amd64" {
+		t.Skipf("skipping test on %s", runtime.GOARCH)
+	}
 	ExitsCorrectly(t, "cap_enter")
 }
 
@@ -253,6 +257,9 @@ func init() {
 }
 
 func TestOpenat(t *testing.T) {
+	if runtime.GOARCH != "amd64" {
+		t.Skipf("skipping test on %s", runtime.GOARCH)
+	}
 	ExitsCorrectly(t, "openat")
 }