Remove assumption that the transition code for transitioning from 64-bit to 32-bit...

Remove assumption that the transition code for transitioning from 64-bit to 32-bit mode is 64 bytes.  Instead compute the size based on the implementation in the ASM file.
Update logic to align new 32-bit stack on a 4 byte boundary
Update logic to prevent one slot overlap between the transition code and the new stack.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10133 6f19259b-4bc3-4df7-8a09-765794883524