objtool: Tidy elf.h

Reorganize elf.h a bit:

- Move the prototypes higher up so they can be used by the inline
  functions.

- Move hash-related code to the bottom.

- Remove the unused ELF_HASH_BITS macro.

No functional changes.

Link: https://lore.kernel.org/r/b1490ed85951868219a6ece177a7cd30a6454d66.1685464332.git.jpoimboe@kernel.org


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>