buildtar: Add ARCH to the archive name

When dealing with multiple sub-arches (like 32- and 64-bit on x86, for
example) generating a bunch of kernel tar archives with the same name
but for different sub-arches could get confusing and error-prone. Also,
the build process could overwrite otherwise unrelated builds and you
probably don't want that. So, add the architecture to the archive name
for more clarity and less shoot-yourself-in-the-foot practices.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>