ARM support is complete, so clean up some of the lit tests:

1. Remove --skip-unimplemented
2. Use --filetype=obj instead of =asm, and remove --assemble
3. Remove --need=allow_dump requirement
4. Remove related TODOs.
5. Fix some CHECK lines because objdump output is slightly different from filetype=asm output.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2384983002 .