powerpc: move __end_rodata to cover arch read-only sections

powerpc has a number of read-only sections and tables that are put after
RO_DATA(). Move the __end_rodata symbol to cover these as well.

Setting memory to read-only at boot is done using __init_begin, change
that to use __end_rodata.

This makes is_kernel_rodata() exactly cover the read-only region, as
well as other things using __end_rodata (e.g., kernel/dma/debug.c).
Boot dmesg also prints the rodata size more accurately.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220916040755.2398112-2-npiggin@gmail.com