This patch implements the probes-based runtime linker interface.
It works as follows:

 - On inferior startup, GDB searches the dynamic linker for a
   number of named probes.

 - If all probes are found, GDB sets a breakpoint on each one.
   Otherwise, the standard function _dl_debug_state is used.

 - When using probes, a per-pspace list is maintained of all
   libraries currently loaded by the inferior.  It's updated
   as necessary every time a solib event stop occurs.

 - When using probes, svr4_current_sos will return a copy of
   the cached list.  When not using probes the entire list
   will be fetched from the inferior as before.

 - If any error occurs, GDB will print a warning and revert to
   the standard interface.