[llvm-readelf]Print filename for multiple inputs and fix formatting regression

This patch addresses two closely related bugs:
https://bugs.llvm.org/show_bug.cgi?id=42930 and
https://bugs.llvm.org/show_bug.cgi?id=42931.

GNU readelf prints the file name for every input unless there is only
one input and that input is not an archive. This patch adds the printing
for multiple inputs. A previous change did it for archives, but
introduced a regression with GNU compatibility for single-output
formatting, resulting in a spurious initial blank line. This is fixed in
this patch too.

Reviewed by: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D65953

llvm-svn: 368435