Fix supplier tests for aarch64

[b-deam]

Because TemplateRenderer.arch defaults to the arch of the system
invoking the tests, when invoked on an aarch64 system, the various
Car instances would throw exceptions ¹ because they were
missing the architecture specific build commands added in
elastic/rally-teams#75

This commit fixes the existing tests to default to x86_64,
and adds two new aarch64 tests.

Footnotes

1. (e.g. esrally.exceptions.SystemSetupError: Car "default" requires config key "system.build_command.arch") ↩

[pquentin]

Thanks, this looks good to me!

I find it sad that Elasticsearch forces us to have so many conditions just because x64 is the default arch and is left out from the build command and generated tarball. Well, that's the way it is.