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-dynamic-vhd/build.yml

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

tools/mkimage-gcp/build.yml

@@ -1 +1,3 @@
 image: mkimage-gcp
+arches:
+  - amd64

tools/mkimage-iso-efi/build.yml

@@ -1,2 +1,5 @@
 image: mkimage-iso-efi
 network: true
+arches:
+  - amd64
+  - arm64

tools/mkimage-qcow2-efi/build.yml

@@ -1,2 +1,5 @@
 image: mkimage-qcow2-efi
 network: true
+arches:
+  - amd64
+  - arm64

tools/mkimage-raw-bios/build.yml

@@ -1,2 +1,3 @@
 image: mkimage-raw-bios
-arches: ["amd64"]
+arches:
+  - amd64

tools/mkimage-raw-efi/build.yml

@@ -1,2 +1,5 @@
 image: mkimage-raw-efi
 network: true
+arches:
+  - amd64
+  - arm64

tools/mkimage-vhd/build.yml

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

tools/mkimage-vmdk/build.yml

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