Using "x86_64" is preferred over "amd64" for arch name

[cboulay]

If we replace amd64 with x86_64 in the second line here then all of our packaged binaries will get the x86_64 suffix instead of amd64. x86_64 is generally preferred because it is less likely to cause confusion about processor manufacturers.

e.g., https://askubuntu.com/questions/608744/is-it-a-problem-to-have-packages-with-amd64-architecture-when-i-have-an-intel