Implement get getSymbolFileOffset with getSymbolAddress.

This has the following advantages:
* Less code.
* The old ELF implementation was wrong for non-relocatable objects.
* The old ELF implementation (and I think MachO) was wrong for thumb.

No current testcase since this is only used from MCJIT and it only uses
relocatable objects and I don't think it supports thumb yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205508 91177308-0d34-0410-b5e6-96231b3b80d8