[rocm] `iree-rocm-target-chip` should take entire GCN arch name when available

[Stella Laurenzo]

Presently, there is string munging like this to infer chip features. While this may be an ok ergonomic thing for users, it is not good for system-system interop.

In my case, I am getting a string like gfx90a:sramecc+:xnack- for the exact device(s) I want to compile for. I should be able to pass that through unmolested.

For the moment, I will chop at the ':' and note this issue.