powerpc: Rearrange copy_thread child stack creation

This makes it a bit clearer where the stack frame is created, and will
allow easier use of some of the stack offset constants in a later
change.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-5-npiggin@gmail.com