Add -dld to library list

This is needed for the new support in Spike for loading extension
libraries.  I've split it out into a seperate commit to avoid the
autoconf re-run mess.
autoconf f6b153b4e08f8e66875992c9fc8957ec29c61241
Implement "spike --extlib=LIBRARY"

This allows a user to build an external library that contains a spike
extension and then load it into spike as opposed to having to
recompile spike every time a new extension is added.  This means I can
install spike once from my package manager and then use it many times,
as opposed to having to git-submodule it into all of my projects.

You still need to pass --extension, all --extlib does is load the
shared library that may or may not contain an extension.