kitakami-common: Set cpu "runtime variant" to cortex-a53

This makes bionic and various art tools that compile/runs cpu-specific
code choose the cortex-a53 variant at runtime. It does not affect
build-time compilation.

Bug: 120773446
Test: bionic benchmarks with and without this change
Change-Id: I670c8acd094090135515688c5f057c2cc08a083f