This is part of AssemblyProgram reduction effort. Details:
- Add Disassembler::Disassemble() and dedup separate PE/ELF versions.
  - Instantiate AssemblyProgram there; replaces old behavior where
    caller (program_detector.cc) instantiates and passes as out param.
- Add virtual Disassembler::GetInstructionGenerator().
  - Remove InstructionGenerator's AssemblyProgram* param; update tests.
- Move InstructionReceptor and InstructionGenerator from AssemblyProgram
  to new file instruction_utils.h.
- Rename ParseAbs32Relocs() to ExtractAbs32Locations(); make common.
- Rename ParseRel32Relocs() to ExtractRel32Locations(); make common.
- Make DisassemblerElf32::abs32_locations_ non-mutable.

BUG=660980

Review-Url: https://codereview.chromium.org/2771753004
Cr-Commit-Position: refs/heads/master@{#459272}