[llvm-objdump] Fix jumptable detection when disassembling Mach-O binaries

- Add LC_SEGMENT_64 handling in getSectionsAndSymbols to be able to find the base segment address from 64-bit Mach-O binaries.
- Add "data in code" detection into the !symbolTableWorked case, extract it into a separate function.
- Fix uninitialized variable usage on BaseSegmentAddress (initialize to 0).
- Add test.

Differential Revision: https://reviews.llvm.org/D65491

llvm-svn: 367578