powerpc: Mark .opd section read-only

.opd section contains function descriptors used to locate
functions in the kernel. If someone is able to modify a
function descriptor he will be able to run arbitrary
kernel function instead of another.

To avoid that, move .opd section inside read-only memory.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3cd40b682fb6f75bb40947b55ca0bac20cb3f995.1634136222.git.christophe.leroy@csgroup.eu