Add a fake bfd to hold linker created dynamic sections

Currently, we hold linker created dynamic sections in an input shared
object, which has its own dynamic sections, when the first input file
from linker is a shared object.  It may lead to conflicts between
linker created dynamic sections and shared object's dynamic sections.
We can use a a fake bfd to hold linker created dynamic sections.
Unfortunately, it doesn't work due to BFD_LINKER_CREATED.  Dynamic
sections in bfd with BFD_LINKER_CREATED may be ignored.
15 files changed