patch 8.2.1487: Travis: installing snd-dummy is not always useful

Problem:    Travis: installing snd-dummy is not always useful.
Solution:   Only install snd-dummy on amd64. (Ozaki Kiichi, closes #6738)
diff --git a/.travis.yml b/.travis.yml
index a8acd3a..7f72e06 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -69,7 +69,11 @@
         fi
     before_script:
       - sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=0
-      - sudo bash ci/load-snd-dummy.sh || true
+      # It appears we can load "snd-dummy" on only amd64.
+      - |
+        if [[ "${TRAVIS_CPU_ARCH}" = amd64 ]]; then
+          sudo bash ci/load-snd-dummy.sh || true
+        fi
       - sudo usermod -a -G audio $USER
       - do_test() { sg audio "sg $(id -gn) '$*'"; }
 
@@ -208,13 +212,13 @@
       env:
         - *normal
         - *shadowopt
-          # Temporarily disabled, always fails
-          #- <<: *linux
-          #  arch: s390x
-          #  name: huge/gcc-s390x
-          #  compiler: gcc
-          #  env: *linux-huge
-          #  services: []
+    # Temporarily disabled, always fails
+    #- <<: *linux
+    #  arch: s390x
+    #  name: huge/gcc-s390x
+    #  compiler: gcc
+    #  env: *linux-huge
+    #  services: []
     - <<: *linux
       arch: arm64
       name: huge/gcc-arm64