[Driver] Allow XRay for more architectures on ELF systems

Codegen OS-agnostic for ELF and the runtime is mostly OS-agnostic. It
seems unnecessary to make restriction.

While here, rewrite test/Driver/XRay/xray-instrument*.c to be more conventional:
specify --target= explicitly instead of relying on the configured default target
(which needs `REQUIRES:`).

I am not sure enumerating every supported architecture is useful, so we just test a few.