powerpc/ftrace: Move ftrace stub used for init text before _einittext

Move the ftrace stub used to cover inittext before _einittext so that it
is within kernel text, as seen through core_kernel_text(). This is
required for a subsequent change to ftrace.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241030070850.1361304-9-hbathini@linux.ibm.com