build: centralize / unify asm-offsets generation

build: centralize / unify asm-offsets generation

Except for an additional prereq Arm and x86 have the same needs here,
and Arm can also benefit from the recent x86 side improvement. Recurse
into arch/*/ only for a phony include target (doing nothing on Arm),
and handle asm-offsets itself entirely locally to xen/Makefile.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>