tools: Clarify the supported architectures

Most of the tools packages are not usable on s390x so
explicitly list them.

Also removed arm64 from mkimage-gcp as GCP does not
support arm machines and fixed a  minor inconsistency
the way the architecture was specified in mkimage-raw-bios.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

tools/mkimage-vhd/build.yml

@@ -1 +1,4 @@
 image: mkimage-vhd
+arches:
+  - amd64
+  - arm64