core dump: user_regset writeback

This makes the user_regset-based core dump code call user_regset writeback
hooks when available.  This is necessary groundwork to allow IA64 to set
CORE_DUMP_USE_REGSET.

Cc: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>