xen: Split x86/debugger.h into common and arch specific parts

xen: Split x86/debugger.h into common and arch specific parts

With all the non-CONFIG_CRASH_DEBUG functionality moved elsewhere, split
x86/debugger.h in two, with the stubs and explanation moved to xen/debugger.h.

In particular, this means that arches only need to provide an $arch/debugger.h
if they implement CONFIG_CRASH_DEBUG, and ARM's stub can be deleted.

Signed-off-by: Bobby Eshleman <bobby.eshleman@gmail.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>