Detect non x86_64 machine in build script.

This change adds functionality to detect whether the
host machine is x86_64, which is the only machine currently supported by
the build script.

Bug: 241967798
Tag: #floss
Test: run ./build.py on non x86_64 and observe script exiting, run on x86_64 and
observe normal behavior.

Change-Id: Ia9837e26c78559e85b00cc8637daa2874cb3d8d8