elf: add ELF_CORE_COPY_KERNEL_REGS()

ELF core dump is used for both user land core dump and kernel crash
dump.  Depending on architecture, register might need to be accessed
differently for userland and kernel.  Allow architectures to define
ELF_CORE_COPY_KERNEL_REGS() and use different operation for kernel
register dump.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>