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

rust/compiler.go

@@ -305,7 +305,7 @@ func (compiler *baseCompiler) compilerDeps(ctx DepsContext, deps Deps) Deps {
 	if !Bool(compiler.Properties.No_stdlibs) {
 		for _, stdlib := range config.Stdlibs {
 			// If we're building for the primary arch of the build host, use the compiler's stdlibs
-			if ctx.Target().Os == android.BuildOs {
+			if ctx.Target().Os == ctx.Config().BuildOS {
 				stdlib = stdlib + "_" + ctx.toolchain().RustTriple()
 			}
 			deps.Stdlibs = append(deps.Stdlibs, stdlib)