powerpc/aout: Fix struct user definition to use user_pt_regs

powerpc/aout: Fix struct user definition to use user_pt_regs

I'm pretty sure this is dead code, it's only used by the a.out core
dump code, and we don't support a.out. We should remove it.

But while it's in the tree it should be using the ABI version of
pt_regs which is called user_pt_regs in the kernel, because the whole
struct is written to the core dump and so its size shouldn't change.

Note this isn't a uapi header so we don't need an ifdef.

Fixes: 1a123cc850b4 ("powerpc: Split user/kernel definitions of struct pt_regs")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>