[llvm-objdump] Support disassembling by symbol name

Summary:
Add a new option -df to llvm-objdump that takes function names
as arguments and instructs the disassembler to only dump those function
contents. Based on code originally written by Bill Nell.

Reviewers: espindola, JDevlieghere

Differential Revision: https://reviews.llvm.org/D44224

llvm-svn: 327164