PR 30569, always call elf_backend_size_dynamic_sections

This largely mechanical patch is preparation for a followup patch.

For quite some time I've thought that it would be useful to call
elf_backend_size_dynamic_sections even when no dynamic objects are
seen by the linker.  That's what this patch does, with some renaming.
There are no functional changes to the linker, just a move of the
dynobj test in bfd_elf_size_dynamic_sections to target backend
functions, replacing the asserts/aborts already there.  No doubt some
of the current always_size_sections functions could be moved to
size_dynamic_sections but I haven't made that change.

Because both hooks are now always called, I have renamed
always_size_sections to early_size_sections and size_dynamic_sections
to late_size_sections.  I condisdered calling late_size_sections plain
size_sections, since this is the usual target dynamic section sizing
hook, but decided that searching the sources for "size_sections" would
then hit early_size_sections and other functions.