only add highlight test BR for ppc64le/x86_64 on EL7+
    
    The highlight package is not available for aarch64 or s390x in EL7+.
    Simplify the conditional (a little) by only listing the 2 known
    architectures where highlight is available for EL7+.  It's not worth
    adding much complexity for a dependency that is only used in 3 tests
    for the gitweb subpackage.