UML: Fix ELF_CORE_COPY_REGS build botch

The earlier crash dump fix on x86_64 depended on patches in -mm which
are intended for post-2.6.23.  Without those, it broke the build when
it went into 2.6.23-rc5.

This changes the field references in ELF_CORE_COPY_REGS back to those
still used in mainline.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>