objtool: Constify 'struct elf *' parameters

In preparation to parallelize certain parts of objtool, map out which uses
of various data structures are read-only vs. read-write.

As a first step constify 'struct elf' pointer passing, most of the secondary
uses of it in find_symbol_*() methods are read-only.

Also, while at it, better group the 'struct elf' handling methods in elf.h.

Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200422103205.61900-2-mingo@kernel.org