elf: Clean up the ELF header file

Fix various style violations in elf.h
- use correct comment format if the comment fits in just one line
- remove the ending period for the one-line comment
- use tab for the indention instead of space
- put the opening brace at the same line of a typedef/union
- remove <name> in a 'typedef struct' for consistency

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>