Replace android.BuildOs with Config.BuildOS

Replace the android.BuildOs constant with Config.BuildOS so that it
can vary based on the product config.

Bug: 190084016
Test: all Soong tests
Change-Id: Ia67f872d8b2ab788747a22e3a9659dc21c9775cd
diff --git a/cc/makevars.go b/cc/makevars.go
index 393170a..8d7a163 100644
--- a/cc/makevars.go
+++ b/cc/makevars.go
@@ -165,7 +165,7 @@
 	sort.Strings(ndkKnownLibs)
 	ctx.Strict("NDK_KNOWN_LIBS", strings.Join(ndkKnownLibs, " "))
 
-	hostTargets := ctx.Config().Targets[android.BuildOs]
+	hostTargets := ctx.Config().Targets[ctx.Config().BuildOS]
 	makeVarsToolchain(ctx, "", hostTargets[0])
 	if len(hostTargets) > 1 {
 		makeVarsToolchain(ctx, "2ND_", hostTargets[1])