ELF: reliably invoke md_elf_section_change_hook()

ELF: reliably invoke md_elf_section_change_hook()

... after any (sub)section change. While certain existing target hooks
only look at now_seg, for a few others it looks as if failing to do so
could have caused anomalies if sub-sections were used. In any event a
subsequent x86 change is going to require the sub-section to be properly
in place at the time the hook is invoked.

This primarily means for obj_elf_section() to pass the new subsection
into change_section(), for it to be set right away (ahead of invoking
the hook).

Also adjust obj_elf_ident() to invoke the hook after all section
changes. (Note that obj_elf_version(), which also changes sections and
then changes them back, has no hook invocation at all so far, so none
are added. Presumably there is a reason for this difference in
behavior.)