Herwig7_i: Compile BB4LH7 shared library only on x86_64
Compile the shared library powhegHerwigBB4L introduced in !72582 (merged) only if CMAKE_HOST_SYSTEM_PROCESSOR
is x86_64
, since including powhegSOforHerwig leads to a failed compilation on aarch64-el9-gcc13-opt
nightly, see this log as reported by @tsulaia. This change was kindly suggested by @averbyts, since POWHEG on ARM is WIP.