clang8 builds are assumed not to use SSE4_2

According to https://gitlab.cern.ch/gaudi/Gaudi/-/blob/v34r0/cmake/GaudiBuildFlags.cmake#L209 we enable sse4.2 instructions when using clang > 5.0, but LbPlatformUtils assumes clang builds never use sse4.2, see https://gitlab.cern.ch/lhcb-core/LbPlatformUtils/-/blob/4.3.2/LbPlatformUtils/init.py#L161