x86: x86 CORE_DUMP_USE_REGSET

This switches x86 to the user_regset-based code for ELF core dumps.
The core dumps come out exactly the same as before.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>