Add an s390 build with clang to the Travis configuration

Since clang builds have been fixed on s390x, including support for
DYNAMIC_ARCH, cover that build type in Travis.

Explicitly request Ubuntu 20.04 (codename focal) to get a recent
LLVM/clang version 10.x and thereby cover all s390x architecture
generations supported in OpenBLAS. Ubuntu 18.10's LLVM/clang 6.x cannot
build the inline assembly in some of the Z13 and Z14 kernels.

LLVM/clang currently does not support OpenMP on s390x, so disable that
in the build.

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>

.travis.yml

@@ -43,6 +43,18 @@ matrix:
         - TARGET_BOX=IBMZ_LINUX
         - BTYPE="BINARY=64 USE_OPENMP=1"
 
+    - <<: *test-ubuntu
+      os: linux
+      dist: focal
+      arch: s390x
+      compiler: clang
+      before_script:
+        - COMMON_FLAGS="DYNAMIC_ARCH=1 TARGET=Z13 NUM_THREADS=32"
+      env:
+        # for matrix annotation only
+        - TARGET_BOX=IBMZ_LINUX
+        - BTYPE="BINARY=64 USE_OPENMP=0 CC=clang"
+
     - <<: *test-ubuntu
       env:
         - TARGET_BOX=LINUX64