Add arch features

Allow architecture toolchains to register "features" supported by the
current variant, and then apply properties from the selected features.
Equivalent to the ARCH_*_HAS_* variables in the combo makefiles.

Change-Id: Ib6823be1c1a52da677d081db9f24336a072eaf39
diff --git a/cc/x86_darwin_host.go b/cc/x86_darwin_host.go
index be40933..4195dae 100644
--- a/cc/x86_darwin_host.go
+++ b/cc/x86_darwin_host.go
@@ -198,11 +198,11 @@
 var toolchainDarwinX86Singleton Toolchain = &toolchainDarwinX86{}
 var toolchainDarwinX8664Singleton Toolchain = &toolchainDarwinX8664{}
 
-func darwinX86ToolchainFactory(archVariant string, cpuVariant string) Toolchain {
+func darwinX86ToolchainFactory(arch common.Arch) Toolchain {
 	return toolchainDarwinX86Singleton
 }
 
-func darwinX8664ToolchainFactory(archVariant string, cpuVariant string) Toolchain {
+func darwinX8664ToolchainFactory(arch common.Arch) Toolchain {
 	return toolchainDarwinX8664Singleton
 }