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/arm64_device.go b/cc/arm64_device.go
index b818ce4..754b276 100644
--- a/cc/arm64_device.go
+++ b/cc/arm64_device.go
@@ -136,7 +136,7 @@
 	return "${arm64Ldflags}"
 }
 
-func arm64ToolchainFactory(archVariant string, cpuVariant string) Toolchain {
+func arm64ToolchainFactory(arch common.Arch) Toolchain {
 	return toolchainArm64Singleton
 }