rust: Fix Host multilib prop for rust_test modules.

NewRustTest will set MultilibBoth true for modules which target
devices, however modules which target both device and host
incorrectly sets MultilibBoth for the host variant. Host tests
can only produce the primary arch target. Therefore, add the
rustTestHostMultilib load hook to set MultilibFirst for the
host variant.

Test: m rust_test module with host_supported: true, check if x86 test
      produced.
Change-Id: I5b04b2a2d898823bcf326336d1d28c46945d47f5

rust/test.go

@@ -210,6 +210,12 @@ func init() {
 
 func RustTestFactory() android.Module {
 	module, _ := NewRustTest(android.HostAndDeviceSupported)
+
+	// NewRustTest will set MultilibBoth true, however the host variant
+	// cannot produce the non-primary target. Therefore, add the
+	// rustTestHostMultilib load hook to set MultilibFirst for the
+	// host target.
+	android.AddLoadHook(module, rustTestHostMultilib)
 	return module.Init()
 }
 
@@ -236,3 +242,16 @@ func (test *testDecorator) compilerDeps(ctx DepsContext, deps Deps) Deps {
 func (test *testDecorator) testBinary() bool {
 	return true
 }
+
+func rustTestHostMultilib(ctx android.LoadHookContext) {
+	type props struct {
+		Target struct {
+			Host struct {
+				Compile_multilib *string
+			}
+		}
+	}
+	p := &props{}
+	p.Target.Host.Compile_multilib = proptools.StringPtr("first")
+	ctx.AppendProperties(p)
+}