Added new files elflink.c, elflink.h, elfcore.h.

Added new files elflink.c, elflink.h, elfcore.h.

Moved some mostly size-independent stuff from elfcode.h to elf.c, adding a data
structure to elfcode.h with some misc data and callback functions.  Added a
pointer to that structure to the target back end data.  More work can be done
here.

Renamed generic elf routines (not cpu-specific stuff) to start with bfd_elf or
_bfd_elf.  Updated most call sites, sometimes defined some macros.

Moved some dynamic linking support code from m68k, sparc, i386 to common files,
using target back end flags to control behavior.  More work can probably be
done here too.

Moved core- and linker-support code out of elfcode.h to elf.c, elfcore.h,
elflink.h, or elflink.c.  Now elfcode.h contains only .o and executable
support, plus #includes of elfcore.h and elflink.h.  Much of the contents of
these other header files can probably still be moved from the .h files to the
.c files, to get compiled only once.

Cleaned up some "gcc -Wall" warnings regarding unused or uninitialized
variables, in generic and cpu-specific code.