objtool/klp: Add --checksum option to generate per-function checksums

objtool/klp: Add --checksum option to generate per-function checksums

In preparation for the objtool klp diff subcommand, add a command-line
option to generate a unique checksum for each function.  This will
enable detection of functions which have changed between two versions of
an object file.

Acked-by: Petr Mladek <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>