Current Thunk16.asm implementation clears reserved bits, which does not follow IA32 SDM. Now the fix is to only clear PAE and PSE bit of CR4 for real mode.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14037 6f19259b-4bc3-4df7-8a09-765794883524