Improve Fedora build host support: architectures.

Instead of blindly calling dpkg, we will now perform an appropriate test
and fall back on uname with some subsequent massaging of uname's output
on amd64 and ARM build hosts.

Based on a patch from Lucas Alvares Gomes

Change-Id: I363addd9f8b4d748d50b69f4509ce627aa5676aa
Merge "Improve Fedora build host support: architectures."