aout, ecoff and som free_cached_info

By inspection the aout, som and ecoff object targets either do not
support _bfd_check_format[bfd_core], or they use different tdata to
the bfd_object support, eg. i386lynx.c and lynx-core.c.  Some use
entirely separate bfd_target vecs for bfd_object vs. bfd_core.  In any
of these cases we either should not or do not need to support bfd_core
in their free_cached_info functions.

This patch removes the bfd_core test from free_cached_info for these
targets.

	* aoutx.h (bfd_free_cached_info): Ignore bfd_core.  Move section
	handling out of tdata test.
	* pdp11.c (bfd_free_cached_info): Likewise.
	* som.c (som_bfd_free_cached_info): Likewise.
	* ecoff.c (_bfd_ecoff_bfd_free_cached_info): Ignore bfd_core.