xen: move perform_gunzip to common

xen: move perform_gunzip to common

The current gunzip code to decompress the Dom0 kernel is implemented in
inflate.c which is included by bzimage.c.

I am looking to doing the same on ARM64 but there is quite a bit of
boilerplate definitions that I would need to import in order for
inflate.c to work correctly.

Instead of copying/pasting the code from x86/bzimage.c, move those
definitions to a new common file, gunzip.c. Export only perform_gunzip
and gzip_check. Leave output_length where it is.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
CC: andrew.cooper3@citrix.com