[dsymutil] Add support for outputting assembly

When implementing the DWARF accelerator tables in dsymutil I ran into an
assertion in the assembler. Debugging these kind of issues is a lot
easier when looking at the assembly instead of debugging the assembler
itself. Since it's only a matter of creating an AsmStreamer instead of a
MCObjectStreamer it made sense to turn this into a (hidden) dsymutil
feature.

Differential revision: https://reviews.llvm.org/D49079

llvm-svn: 336561