build: add core_64_bit.mk

Add a new product, core_64_bit.mk, that products can inherit from
to configure zygote and the rest of the build system for a standard
64-bit product.

Make the 64-bit emulator targets for arm64, mips64, and x86_64
inherit from it.

Change-Id: I7e809264db39472f554cd5290529f3d6499345d4