Do not show unrelated "-m is missing" error message.

Previously, "-m is missing" error message is shown if you pass a
nonexistent file or don't pass any file at all to lld, as shown below:

  $ ld.lld nonexistent.o
  ld.lld: error: cannot open nonexistent.o: No such file or directory
  ld.lld: error: target emulation unknown: -m or at least one .o file required

This patch eliminates the second error message because it's not related
and even inaccurate (you passed a .o file though it didn't exist).

llvm-svn: 334024