finally added support for floating point exception handling on MacOS X.

The issue is that OS X uses the SSE unit for all FP math by default,
not the x87 FP unit, so one has to use _MM_SET_EXCEPTION_MASK.


git-svn-id: http://root.cern.ch/svn/root/trunk@38602 27541ba8-7e3a-0410-8455-c3a389f83636