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/build/Android.common.mk b/build/Android.common.mk
index f22eb37..30f730c 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -37,6 +37,12 @@
 ART_BUILD_HOST_NDEBUG ?= $(WITH_HOST_DALVIK)
 ART_BUILD_HOST_DEBUG ?= $(WITH_HOST_DALVIK)
 
+ifeq ($(BUILD_HOST_64bit),)
+ART_HOST_ARCH := x86
+else
+ART_HOST_ARCH := x86_64
+endif
+
 ifeq ($(ART_BUILD_TARGET_NDEBUG),false)
 $(info Disabling ART_BUILD_TARGET_NDEBUG)
 endif