Replace `run-dysutil` option with more general `split-debuginfo` option
that works on all platforms.

Signed-off-by: David Wood <david.wood@huawei.com>

Signed-off-by: David Wood <david.wood@huawei.com>

Small fix that prevents `thorin` from running on platforms where it
definitely shouldn't be running.

Signed-off-by: David Wood <david.wood@huawei.com>

Temporarily, only enable split debuginfo on Windows if not building with
the boostrap compiler as there is a bug that isn't fixed in the
bootstrap compiler which would result in `thorin` being run on Windows.

Signed-off-by: David Wood <david.wood@huawei.com>