Use correct path separator for satellite assemblies in cross builds (#4976)

When creating a bundle on Windows to run on Android, macOS/iOS or Linux we need
to use the target system's path separator when storing satellite assemblies in
the bundle instead of the host system's one. Failing to do so will embed (on
Windows) the satellite assemblies with paths like

   fr_FR\myassembly.resources.dll

while at runtime Mono will try to look up the assembly by

   fr_FR/myassembly.resources.dll

and fail to find it, obviously. The `--style` parameter is now consulted to see
what path separator character should be used.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=55603