xen: asm-generic support

Some headers are shared between individual architectures or are empty.
To avoid duplication of these headers, asm-generic is introduced.

With the following patch, an architecture uses generic headers
mentioned in the file arch/$(ARCH)/include/asm/Makefile

To use a generic header is needed to add to
arch/$(ARCH)/include/asm/Makefile :
	generic-y += <name-of-header-file.h>

For each mentioned header in arch/$(ARCH)/include/asm/Makefile,
the necessary wrapper in arch/$(ARCH)/include/generated/asm will be
generated.

As the base Makefile.asm-generic from Linux kernel was taken.
( 06c2afb862f9da8 "Linux 6.5-rc1" ).

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

.gitignore

@@ -267,6 +267,7 @@ xen/arch/*/efi/efi.h
 xen/arch/*/efi/pe.c
 xen/arch/*/efi/runtime.c
 xen/arch/*/include/asm/asm-offsets.h
+xen/arch/*/include/generated
 xen/build-dir-cppcheck/
 xen/common/config_data.S
 xen/common/config.gz