config_targetos is now empty and can be removed; its use in sourcesets
that do not involve target-specific files can be replaced with an empty
dictionary.

In fact, at this point *all* sourcesets that do not involve
target-specific files are just glorified mutable arrays.  Enforce that
they never test for symbols in "when:" by computing the set of files
without "strict: false".

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>