Fail if non-allowlisted host system tool is used.

This CL improves the clarity of the non-hermetic nature of using
HostSystemTools, and make the build fail if a caller tries to use a host
system tool that isn't allowlisted in config.go.

The only caller seems to be for xcrun, which is a special case that is
pretty hard to remove.

Test: TH presubmit
Change-Id: Icd3e7330406a08c00ac9682e37e2d42669f2e6b7
diff --git a/cc/config/x86_darwin_host.go b/cc/config/x86_darwin_host.go
index 81c907d..d7ff580 100644
--- a/cc/config/x86_darwin_host.go
+++ b/cc/config/x86_darwin_host.go
@@ -135,7 +135,7 @@
 
 func getMacTools(ctx android.PackageVarContext) *macPlatformTools {
 	macTools.once.Do(func() {
-		xcrunTool := ctx.Config().HostSystemTool("xcrun")
+		xcrunTool := ctx.Config().NonHermeticHostSystemTool("xcrun")
 
 		xcrun := func(args ...string) string {
 			if macTools.err != nil {