Fix error message for unknown -format argument.

-format=<foo>, -format <foo> and -b <foo> are all the same.
Previous code was intended to produce an error message with the
same spelling as given from the command line, but it actually
always printed out this string: "unknown -format= value:".
This is probably more confusing than "unknown -format value:".
So I changed the message.

llvm-svn: 284693