Don't build vixl unless WITH_HOST_DALVIK is defined.
Don't build vixl unless WITH_HOST_DALVIK is defined.

This works around build issues with OS/X.

Change-Id: Iac38cf6d389d4728591d447131195240766c5cea

android-ia/platform_external_vixl,xin3liang/platform_external_vixl,geekboxzone/mmallow_external_vixl,CyanogenMod/android_external_vixl,xin3liang/platform_external_vixl,android-ia/platform_external_vixl,geekboxzone/mmallow_external_vixl,CyanogenMod/android_external_vixl,geekboxzone/mmallow_external_vixl,xin3liang/platform_external_vixl,android-ia/platform_external_vixl,geekboxzone/lollipop_external_vixl,geekboxzone/lollipop_external_vixl,CyanogenMod/android_external_vixl,geekboxzone/lollipop_external_vixl