Change how FXSAVE and FXRSTOR are done, so as to avoid pushing the XMM
register contents themselves through the helper functions.  This
avoids the false positives reported in #291310.



git-svn-id: svn://svn.valgrind.org/vex@2949 8f6e269a-dfd6-0310-a8e1-e2731360e62c