Add method to determine variations from a Target

The arch variants are hardcoded in every module type.  Refactor
them out into a Target.Variations() method in preparation for
splitting the arch mutator into two, which will require using
different variations.

Test: m checkbuild
Change-Id: I28ef7cd5168095ac888fe77f04e27f9ad81978c0
diff --git a/android/arch.go b/android/arch.go
index 348b064..5887744 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -22,9 +22,12 @@
 	"strconv"
 	"strings"
 
+	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
 
+const COMMON_VARIANT = "common"
+
 var (
 	archTypeList []ArchType
 
@@ -36,7 +39,7 @@
 	X86_64 = newArch("x86_64", "lib64")
 
 	Common = ArchType{
-		Name: "common",
+		Name: COMMON_VARIANT,
 	}
 )
 
@@ -702,11 +705,23 @@
 }
 
 func (target Target) String() string {
-	variant := ""
+	return target.ArchVariation()
+}
+
+func (target Target) ArchVariation() string {
+	var variation string
 	if target.NativeBridge {
-		variant = "native_bridge_"
+		variation = "native_bridge_"
 	}
-	return target.Os.String() + "_" + variant + target.Arch.String()
+	variation += target.Arch.String()
+
+	return target.Os.String() + "_" + variation
+}
+
+func (target Target) Variations() []blueprint.Variation {
+	return []blueprint.Variation{
+		{Mutator: "arch", Variation: target.ArchVariation()},
+	}
 }
 
 // archMutator splits a module into a variant for each Target requested by the module.  Target selection