powerpc: Set eflags correctly for ELF ABIv2 core dumps.

We leave it at zero (though it could be 1) for old tasks.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>