Fixed and refactored 64b host build

Add a global variable ART_HOST_ARCH in build/Android.common.mk that
will be set to x86 or x86_64 depending on BUILD_HOST_64bit. This is
then used as the instruction set for dex2oat.

Change-Id: Icec8ef9139f780314a3ff325a729750e65d6d8da
diff --git a/test/Android.mk b/test/Android.mk
index f4a0426..bae4680 100644
--- a/test/Android.mk
+++ b/test/Android.mk
@@ -115,7 +115,7 @@
 	$(hide) rm /tmp/test-art-target-oat-$(1)
 
 $(HOST_OUT_JAVA_LIBRARIES)/oat-test-dex-$(1).odex: $(HOST_OUT_JAVA_LIBRARIES)/oat-test-dex-$(1).jar $(HOST_CORE_IMG_OUT) | $(DEX2OAT)
-	$(DEX2OAT) --runtime-arg -Xms16m --runtime-arg -Xmx16m --boot-image=$(HOST_CORE_IMG_OUT) --dex-file=$(PWD)/$$< --oat-file=$(PWD)/$$@ --instruction-set=$(HOST_ARCH) --host --android-root=$(HOST_OUT)
+	$(DEX2OAT) --runtime-arg -Xms16m --runtime-arg -Xmx16m --boot-image=$(HOST_CORE_IMG_OUT) --dex-file=$(PWD)/$$< --oat-file=$(PWD)/$$@ --instruction-set=$(ART_HOST_ARCH) --host --android-root=$(HOST_OUT)
 
 .PHONY: test-art-host-oat-default-$(1)
 test-art-host-oat-default-$(1): $(HOST_OUT_JAVA_LIBRARIES)/oat-test-dex-$(1).odex test-art-host-dependencies