Support more architectures

[hhorak]

Not all SCLs are x86_64, for example, devtoolset v6 and v7 are already available as aarch64 and ppc64le. However, scl.org hard-codes x86_64 on some cases. We should be able to either list all supported arches, or use some architecture agnostic strings.

[qhaas]

This issue seems to have been addressed in this centos7 bug report, but there are still far fewer packages built for ppc64le than x86-64 in the SCL.