Include `amd64` in 64-bit variants

This is known to be used on Debian and FreeBSD. Although Ohai normalizes 
all Linux platforms to `x86_64` it appears `node[‘kernel’][‘machine’]` 
still returns `amd64` on FreeBSD.
Merge pull request #67 from schisamo/schisamo/amd64-is-64-bit

Include `amd64` in 64-bit variants