Add a 32-bit x86_64 arch variant

In make, the current 32-bit configuration for generic x86_64 targets
inherits some variables (SSE4 support) from the 64-bit configuration,
and overrides the make variables used for other configurations (SSSE3).

We don't have that problem in Soong, but this is the configuration
matching what Make is using.

Change-Id: Ia934ff40f9388f9c202a0fe0608e7b45b77a2714
diff --git a/common/arch.go b/common/arch.go
index 8ad5087..4176cd1 100644
--- a/common/arch.go
+++ b/common/arch.go
@@ -186,6 +186,8 @@
 			Ivybridge   interface{} `blueprint:"filter(android:\"arch_variant\")"`
 			Sandybridge interface{} `blueprint:"filter(android:\"arch_variant\")"`
 			Silvermont  interface{} `blueprint:"filter(android:\"arch_variant\")"`
+			// Generic variant for X86 on X86_64
+			X86_64      interface{} `blueprint:"filter(android:\"arch_variant\")"`
 
 			// X86 arch features
 			Ssse3  interface{} `blueprint:"filter(android:\"arch_variant\")"`
@@ -955,6 +957,7 @@
 		{"x86", "ivybridge", "", []string{"x86"}},
 		{"x86", "sandybridge", "", []string{"x86"}},
 		{"x86", "silvermont", "", []string{"x86"}},
+		{"x86", "x86_64", "", []string{"x86"}},
 		{"x86_64", "", "", []string{"x86_64"}},
 		{"x86_64", "haswell", "", []string{"x86_64"}},
 		{"x86_64", "ivybridge", "", []string{"x86_64"}},