Add an s390 build with clang to the Travis configuration

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>