Update LTO plugin interface

[Cary Coutant]

> I propose that we add a new linker option: --plugin-callback.  At each
> point where this option appears on the command line, the linker will
> call a new plugin callback entry point.  The LTO plugin will replace the
> all_symbols_read callback with this one.  We will have the gcc driver
> run the linker more or less like this:
>
> object-files -lc -lgcc --plugin-callback -lc -lgcc crtend.o crtn.o

I'm not sure how this is any better than the pass-through option we're
already using. That just has the plugin re-inject those libraries that
you have placed after the --plugin-callback option. The crtend.o and
crtn.o files could be handled by having the plugin claim them and
re-inject them at the end.

For every new routine that the gcc backend generates a new call to, it
ought to know which library that routine is defined in, and should be
able to add that library after the generated object(s) during the
all-symbols-read callback. We really don't want to support arbitrary
interposition at that point, because a user-supplied replacement might
invalidate some assumptions that were made during optimization.

-cary