Get execution platforms and aarch64-linux supported properly

[thoughtpolice]

I think I have an OK understanding of how execution platforms, etc should work. But not all the targets work on my laptop, particularly renode ones, so we need to conditionalize them with select(). Also, buildbarn probably doesn't work either... We'll have to figure this out.

[thoughtpolice]

ebb5efdce856d1da0123537335c0e85b13deb72f