Extend the CPU ABI specification mechanism.

Add a (read only) system property that is a comma
separated list of ABIs supported by the device in order
of preference. For example, typical arm-v8 device might
define:

ro.cpu.abilist = arm64-v8a,armeabi-v7a,armeabi

For most purposes, a single flattened list like the above is
probably more useful than the parallel system of variables
TARGET_CPU_ABI{2} / TARGET_2ND_ARCH_CPU_ABI{2} that we use
in the build system.

Change-Id: If9102669ad9f5f8fd89a8bcc5bf88cca1acadc3c