Support Universal dSYM files in llvm-objdump

Summary:
Commonly programmers use llvm-objdump to disassemble Mach-O target
binaries with Mach-O dSYMS. While llvm-objdump allows programmers to
disassemble Universal binaries, it previously did not recognize
Universal dSYM files. This change updates llvm-objdump to support
passing in Universal files via the -dsym option. Now, when
disassembling a Mach-O file either as a stand alone file or as an entry
in a Universal binariy, llvm-objdump will search through a Universal
dSYM for a Mach-O matching the architecture flag of the file being
disassembled.

Reviewers: pete, lhames

Reviewed By: pete

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62642

llvm-svn: 362121