Hide RdtscClock.h to non x86_64 builds
When testing the standalone build of public headers on ARM, the header Gaudi/Timers/RdtscClock.h
fails because it is not supported on non-x86_64.
This MR tweaks the configuration to make sure Gaudi/Timers/RdtscClock.h
is installed only for x86_64.
This MR, accidentally, fixes a minor inconsistency in some GaudiToolbox.cmake
doc.