[ELF] Add LinkingContext to the ELFReader.

This adds the LinkingContext parameter to the ELFReader. Previously the flags in
that were needed in the Context was passed to the ELFReader, this made it very
hard to access data structures in the LinkingContext when reading an ELF file.

This change makes the ELFReader more flexible so that required parameters can be
grabbed directly from the LinkingContext.

Future patches make use of the changes.

There is no change in functionality though.

llvm-svn: 228905