yaml2obj: add -format=<fmt> to choose input YAML interpretation

See the comment in yaml2obj.cpp for why this is currently needed.
Eventually we can get rid of this, but for now it is needed in order to
make forward progress with adding ELF support, and should be
straightforward to remove later.

Also, preserve the default of COFF, to avoid breaking existing tests.
This policy can easily be changed later though.