powerpc/64: Change the way relocation copy is calculated

With a subsequent patch to put text into different sections,
(_end - _stext) can no longer be computed at link time to determine
the end of the copy. Instead, calculate it at runtime with
(copy_to_here - _stext) + (_end - copy_to_here).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>