[llvm-objdump] Improve newline consistency between different pieces of information

When dumping multiple pieces of information (e.g. --all-headers),
there is sometimes no separator between two pieces.
This patch uses the "\nheader:\n" style, which generally improves
compatibility with GNU objdump.

Note: objdump -t/-T does not add a newline before "SYMBOL TABLE:" and "DYNAMIC SYMBOL TABLE:".
We add a newline to be consistent with other information.

`objdump -d` prints two empty lines before the first 'Disassembly of section'.
We print just one with this patch.

Differential Revision: https://reviews.llvm.org/D101796