refine VariablePei driver to unify the algorithm of access VariableIndexTable among...

refine VariablePei driver to unify the algorithm of access VariableIndexTable among various archs.
1. record the distance of two neighboring VAR_ADDED type variables rather than the offset of each variable. As the field recording this info is UINT16 width, the latter causes in IA32/X64 platform, it can only cache those variables from offset 0 to offset 2^16; in IPF platform, from offset 0 to offset 2^18(extend the scope by left-shift the offset two bits).
when taking the former algorithm, the max range of caching variable is from offset 0 to offset 122*(2^16)

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@8625 6f19259b-4bc3-4df7-8a09-765794883524