Switch shared library default backend to solib-target.c

[Pedro Alves <pedro at codesourcery dot com>]

This patch changes GDB to default to enabling target
reported shared libraries.  I've applied it.

It does that by using solib-target.c by default, instead of
solib-null.c.  The patch actually gets rid of solib-null.c entirely.
This means that for example, a generic elf gdb build (e.g., arm-eabi, without a
default OSABI), can be used to debug random RTOSs that report loaded
shared libraries using the "qXfer:libraries:read" remote protocol
packet.

Since solib-target.c behaves exactly the same as solib-null.c
when the target doesn't implement the TARGET_OBJECT_LIBRARIES
xfer, this is always either a win, or a nop.

-- 
Pedro Alves