The current behavior is to assume a machine is 64-bit unless a regex
for i386/i686 is matched. This causes alternative 32-bit architectures
(e.g. 32-bit ARM) to be misdetected as 64-bit.

This change attempts to better detect 64-bit architectures:
* Any machine type including "64" is assumed to be 64-bit (e.g.
x86_64, aarch64)
* Some known 64-bit machine types without "64" are explicitly matched
(e.g. armv8l, s390x)
* Anything else is assumed to be 32-bit (e.g. i686, armv7l)