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
