preserve mxcsr in mcontext

This was "PR 273323: 64-bit DR may need to save and restore MxCsr on context switches" but was never filed in a newer tracker.

Today DR does not preserve mxcsr, neither its volatile nor non-volatile portions. This is a potential problem in both directions: DR messing up the app, and the app messing up DR, esp with DR code using more and more xmm operations as time goes by and the compilers we build DR with target newer processors.