syscall, internal/routebsd: use hw.supported_arches on FreeBSD

Reading textual representation of the kernel config file to search for
the "machine" config clause is awful, not to mention that kern.conftxt
might be disabled at all.

Use sysctl hw.supported_archs to detect 32bit binary running on amd64
host.

src/internal/routebsd/sys_freebsd.go

@@ -5,6 +5,7 @@
 package routebsd
 
 import (
+	"internal/stringslite"
 	"syscall"
 	"unsafe"
 )
@@ -25,26 +26,9 @@ func probeRoutingStack() (int, map[int]*wireFormat) {
 	// to know the underlying kernel's architecture because the
 	// alignment for routing facilities are set at the build time
 	// of the kernel.
-	conf, _ := syscall.Sysctl("kern.conftxt")
-	for i, j := 0, 0; j < len(conf); j++ {
-		if conf[j] != '\n' {
-			continue
-		}
-		s := conf[i:j]
-		i = j + 1
-		if len(s) > len("machine") && s[:len("machine")] == "machine" {
-			s = s[len("machine"):]
-			for k := 0; k < len(s); k++ {
-				if s[k] == ' ' || s[k] == '\t' {
-					s = s[1:]
-				}
-				break
-			}
-			if s == "amd64" {
-				align = 8
-			}
-			break
-		}
+	arches, _ := syscall.Sysctl("hw.supported_archs")
+	if stringslite.Index(arches, "amd64") >= 0 {
+		align = 8
 	}
 	ifm := &wireFormat{extOff: 16, bodyOff: sizeofIfMsghdr}
 	ifm.parse = ifm.parseInterfaceMessage

src/syscall/route_freebsd.go

@@ -4,28 +4,21 @@
 
 package syscall
 
-import "unsafe"
+import (
+	"internal/stringslite"
+	"unsafe"
+)
 
 func init() {
-	conf, _ := Sysctl("kern.conftxt")
-	for i, j := 0, 0; j < len(conf); j++ {
-		if conf[j] != '\n' {
-			continue
-		}
-		s := conf[i:j]
-		i = j + 1
-		if len(s) > len("machine") && s[:len("machine")] == "machine" {
-			s = s[len("machine"):]
-			for k := 0; k < len(s); k++ {
-				if s[k] == ' ' || s[k] == '\t' {
-					s = s[1:]
-				}
-				break
-			}
-			freebsdConfArch = s
-			break
+	machine, _ := Sysctl("hw.machine")
+	if machine == "i386" {
+		arches, _ := Sysctl("hw.supported_archs")
+		amd64 := "amd64"
+		if stringslite.Index(arches, amd64) >= 0 {
+			machine = amd64
 		}
 	}
+	freebsdConfArch = machine
 }
 
 func (any *anyMessage) toRoutingMessage(b []byte) RoutingMessage {