Sort NDK_MIGRATED_LIBS in make_vars

Otherwise this was changing every time, and causing Kati to find
make_vars-*.mk different every time.

Test: with https://android-review.googlesource.com/274439
Change-Id: I731b3b2fd434314bf6e8b7c2ec5310b9623512a5

cc/makevars.go

@@ -17,6 +17,7 @@ package cc
 import (
 	"fmt"
 	"path/filepath"
+	"sort"
 	"strings"
 
 	"android/soong/android"
@@ -51,6 +52,7 @@ func makeVarsProvider(ctx android.MakeVarsContext) {
 	ctx.StrictRaw("SRC_HEADERS", strings.Join(includes, " "))
 	ctx.StrictRaw("SRC_SYSTEM_HEADERS", strings.Join(systemIncludes, " "))
 
+	sort.Strings(ndkMigratedLibs)
 	ctx.Strict("NDK_MIGRATED_LIBS", strings.Join(ndkMigratedLibs, " "))
 
 	hostTargets := ctx.Config().Targets[android.Host]