Enable sse 4.2 by default for clang 6.0 and newer as well
Currently building for x86_64-centos7-clang8-opt
does not automatically enable SSXE4.2, unlike x86_64-centos7-gcc9-opt
which does. This MR adds parity so both now are defined with the same vectorisation level enabled.
Hopefully will help to remove some unnecessary diffs between the gcc9 and clang8 test results in LHCb nightlies.
Edited by Christopher Rob Jones