GitLab

This changes android to use the target_arch build arg for setting the
target architecture instead of cpu_arch.

Some benefits of this:
non-target toolchains have knowledge of the target architecture
(required for the relocation packer host executable)
allows us to have a default target_arch different from build_cpu_arch

BUG=359249,344767

Review URL: https://codereview.chromium.org/634403004

Cr-Commit-Position: refs/heads/master@{#300136}