Basic runtime dynamic loading capabilities added to ELFObjectFile, implemented

Basic runtime dynamic loading capabilities added to ELFObjectFile, implemented
in a subclass named DyldELFObject. This class supports rebasing the object file
it represents by re-mapping section addresses to the actual memory addresses
the object was placed in. This is required for MC-JIT implementation on ELF with
debugging support.

Patch reviewed on llvm-commits.

Developed together with Ashok Thirumurthi and Andrew Kaylor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148653 91177308-0d34-0410-b5e6-96231b3b80d8