[llvm-objdump] - Change the output for --all-headers.

This is for https://bugs.llvm.org/show_bug.cgi?id=40008,

it starts printing the file headers when --all-headers is given and
do a minor cosmetic change.

Differential revision: https://reviews.llvm.org/D56588

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351006 91177308-0d34-0410-b5e6-96231b3b80d8