Fixed issues with X64 clang, and also make StackSwitch push a zero on the new stack to prevent a stack unwind into memory that is no longer valid.
signed-off-by: andrewfish
reviewed-by: lgao4
reviewed-by: mdkinney
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12007 6f19259b-4bc3-4df7-8a09-765794883524
andrewfish