Add a base makefile for 64 bit only builds.

This makefile specifies a configuration that only starts a 64 bit
zygote (as opposed to both 64/32, as standard 64 bit builds do).
It should be inherited by products that only supports 64 bit apps.

Bug: 72810053
Test: Manual, using product in https://android-review.googlesource.com/608276
Change-Id: Ib88eccf8d4c575bfd93d6edb130348aeca4ae3f8