Porting the changes to support generic dictionaries lookups in R2R images to the...

Porting the changes to support generic dictionaries lookups in R2R images to the JIT32 implementation, to support the x86 architecture.

All of the changes in importer.cpp were ported from JIT\importer.cpp, which was merged to the coreclr master branch last Friday evening (reviewed and signed-off in a PR on GitHub).

The remaining changes include:
         1) Adjusting #ifdefs to enable the x86 scenario
         2) Implementation of the R2R stub for generic dictionary lookups
         3) Small change in prestub.cpp required for x86 to work correctly

[tfs-changeset: 1605906]