This matches LLVM's -filetype={obj,asm} options, and also adds -filetype=iasm to get textual rendering with the integrated assembler.

It also removes the old illegal combination of -ias=0 -elf-writer.

BUG= none
R=jvoung@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/920953002