powerpc/Kconfig: Enable STRICT_KERNEL_RWX for some configs

All code that patches kernel text has been moved over to using
patch_instruction() and patch_instruction() is able to cope with the
kernel text being read only.

The linker script has been updated to ensure the read only data ends
on a large page boundary, so it and the preceding kernel text can be
marked R_X. We also have implementations of mark_rodata_ro() for Hash
and Radix MMU modes.

There are some corner-cases missing when the kernel is built
relocatable, so for now make it depend on !RELOCATABLE.

There's also a temporary workaround to depend on !HIBERNATION to avoid
a build failure, that will be removed once we've merged with the PM
tree.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
[mpe: Make it depend on !RELOCATABLE, munge change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>