Current IPF version SwitchStack implementation uses loadrs instruction to restore...

Current IPF version SwitchStack implementation uses loadrs instruction to restore dirty partition of register stack from the backing store, but there is no guarantee to make the backing store valid before. So  add flushrs before loadrs to make sure backing store valid.

Signed-off-by: Li Elvin <elvin.li@intel.com>
Reviewed-by: Li Kevin <kevin.y.li@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13183 6f19259b-4bc3-4df7-8a09-765794883524