Architecture support should not be hardcoded in the tool

Summary

Seems the architectures buildstream supports are now hardcoded in the code (It used to not be the case), so if you try to build for some arch that is not in the list the build fail

Steps to reproduce

• Try to build freedesktop-sdk for powerpc64le, for example

What is the current bug behavior?

• Build fails reporting the arch is not supported

What is the expected correct behavior?

• While the easy solution would be to add the new arch to the list inside buildstream code, I think this is something independen from buildstream code and should not be harcoded inside buildstream, but problably in a external file on the specific project