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.

llvm-svn: 205508