Change crosstest.py's -elf argument to the now-standard -filetype and just pass it through to llvm2ice.  This also fixes a leftover -elf-writer argument that should have been changed to -filetype=obj.

It would be nice to change all the explicit -threads=0 to -threads=1 in the lit tests, but that uncovers too many assumptions about dump/emit output order, or ordering between function and global data emission.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/924023002