don't build host dalvik on native x86_64

Much of libcore is horribly 32-bit specific with jint pointers.

Change-Id: I1f5e3ab5b0633ad83c33e64e1fb6b19e55102ee3
disable dexpreopt on x86_64 as well

Change-Id: I3af659e2c51f37b976ba4b0a4e544fce3ff41019