Switch x86_64 emulator to 64 bit primary.

Also siezes the opportunity to remove the hardcoded
TARGET_PREFER_32_BIT_APPS directing in core_64_bit.mk.
This avoids the need for further hacks but needs supporting
changes to a few apps to force them to 32 bit.

Change-Id: I36ba9e5f5b08dd87d6a4afc27961a436306eed99