coredump: default CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS=y

The userland ELF tools have been coping with partial-segments core files
for a few years now.  Multiple distro builds are now setting this option.
It behooves everyone who ever deals with core files to have more info
dumped in there, especially as more and more people's compilers are
producing build IDs.  Make it the default.

Anyone using older tools confused by these core files can configure this
option off, or just change /proc/PID/coredump_filter after boot.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>