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>