Use musl rust prebuilts for USE_HOST_MUSL=true

Building rust code with musl enabled can load proc macro crates built
against musl into rustc, so we have to use a version of rustc also
built against musl.

Bug: 216192129
Test: m USE_HOST_MUSL=true host-native
Change-Id: Icd23b542a4b64a975850a8a4bb4b69183cc6c2c6
diff --git a/rust/compiler.go b/rust/compiler.go
index bcd58c8..365f1e0 100644
--- a/rust/compiler.go
+++ b/rust/compiler.go
@@ -372,7 +372,7 @@
 	if !Bool(compiler.Properties.No_stdlibs) {
 		for _, stdlib := range config.Stdlibs {
 			// If we're building for the build host, use the prebuilt stdlibs
-			if ctx.Target().Os == android.Linux || ctx.Target().Os == android.Darwin {
+			if ctx.Host() && !ctx.Target().HostCross {
 				stdlib = "prebuilt_" + stdlib
 			}
 			deps.Stdlibs = append(deps.Stdlibs, stdlib)