blacklist ocExternal for calc's threaded calculations

Opcode ocExternal is used for functions implemented as UNO calls,
which has a number of problems:
- ooo#118213-2 contains GETEOMONTH(), which maps to ocExternal, which
  calls AnalysisAddIn::getEomonth() in scaddins, which ends up calling
  ScModelObj::getPropertyValue(), which deadlocks on SolarMutex
- it uses ScUnoAddInCollection class, which uses delayed initialization
  (even though it's created on-demand), which is not thread-safe; however,
  it seems that the initialization is generally done already while loading
  a file, so this is possibly in practice safe
- who knows what all kinds of race conditions there are in all the functions
  this may call via UNO

Change-Id: I80c4264102b8bc492853852c2c12e5cd2a8ea99e
Reviewed-on: https://gerrit.libreoffice.org/55382Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>

sc/source/core/tool/token.cxx

@@ -1321,7 +1321,8 @@ void ScTokenArray::CheckForThreading( OpCode eOp  )
         ocCell,
         ocInfo,
         ocStyle,
-        ocDBSum
+        ocDBSum,
+        ocExternal
     });
 
     // We only call this if it was already disabled