powerpc: Split user/kernel definitions of struct pt_regs

We use a shared definition for struct pt_regs in uapi/asm/ptrace.h.
That means the layout of the structure is ABI, ie. we can't change it.

That would be fine if it was only used to describe the user-visible
register state of a process, but it's also the struct we use in the
kernel to describe the registers saved in an interrupt frame.

We'd like more flexibility in the content (and possibly layout) of the
kernel version of the struct, but currently that's not possible.

So split the definition into a user-visible definition which remains
unchanged, and a kernel internal one.

At the moment they're still identical, and we check that at build
time. That's because we have code (in ptrace etc.) that assumes that
they are the same. We will fix that code in future patches, and then
we can break the strict symmetry between the two structs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>