objtool: Move macros describing structures to arch-dependent code

Some macros are defined to describe the size and layout of structures
exception_table_entry, jump_entry and alt_instr. These values can vary
from one architecture to another.

Have the values be defined by arch specific code.

Suggested-by: Raphael Gault <raphael.gault@arm.com>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Julien Thierry <jthierry@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>