Simplify arch target handling

Soong's multi-architecture building has grown complex, with the
combination of HostOrDevice+HostType+Arch necessary to determine how to
build a variant of a module, and three separate mutators to split each
into its variations.

Combine HostOrDevice+HostType into Os, which will be Linux, Darwin,
Windows, or Android.  Store Os+Arch as a single Target.

Change-Id: I92f2e2dac53617d595a35cc285d2bd348baa0fbd
diff --git a/cc/x86_darwin_host.go b/cc/x86_darwin_host.go
index f3cf1c9..987b1dd 100644
--- a/cc/x86_darwin_host.go
+++ b/cc/x86_darwin_host.go
@@ -280,6 +280,6 @@
 }
 
 func init() {
-	registerHostToolchainFactory(android.Darwin, android.X86, darwinX86ToolchainFactory)
-	registerHostToolchainFactory(android.Darwin, android.X86_64, darwinX8664ToolchainFactory)
+	registerToolchainFactory(android.Darwin, android.X86, darwinX86ToolchainFactory)
+	registerToolchainFactory(android.Darwin, android.X86_64, darwinX8664ToolchainFactory)
 }