Allow urls in llvm_extra_distributions (#501)

In extra_llvm_distributions allow URLs and absolute filenames. If
absolute filenames are used, then we turn them into "file:" urls which
are also understood.

If strip_prefix is not given, then we compute it.

Extend tests to verify llvm_extra_distributions actually work.

Also stop reporting aliasing of sparc64 and sparcv9 as error.

Next step:
* Drastically reduce golden output since we no longer need compatibility
with original golden data.
* Drastically reduce test run time.

Author: helly25