powerpc: Implement set_memory_rox()

Same as x86 and s390, add set_memory_rox() to avoid doing
one pass with set_memory_ro() and a second pass with set_memory_x().

See commit 60463628 ("x86/mm: Implement native set_memory_rox()")
and commit 22e99fa5 ("s390/mm: implement set_memory_rox()") for
more information.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/dc9a794f82ab62572d7d0be5cb4b8b27920a4f78.1708078316.git.christophe.leroy@csgroup.eu