Introduce the portSOFTWARE_BARRIER macro which thus far is only used by the Win32...

Introduce the portSOFTWARE_BARRIER macro which thus far is only used by the Win32 demo to hold execution up in case a simulated interrupt is executing simultaneously.  That should never happen as all threads should execute on the same core, but we have had numerous reports that this and other Win32 port changes we have made fixed these issues - although we have not been able to replicate them ourselves.