powerpc: Activate CONFIG_STRICT_KERNEL_RWX by default

CONFIG_STRICT_KERNEL_RWX should be set by default on every
architectures (See https://github.com/KSPP/linux/issues/4)

On PPC32 we have to find a compromise between performance and/or
memory wasting and selection of strict_kernel_rwx, because it implies
either smaller memory chunks or larger alignment between RO memory
and RW memory.

For instance the 8xx maps memory with 8M pages. So either the limit
between RO and RW must be 8M aligned or it falls back or 512k pages
which implies more pressure on the TLB.

book3s/32 maps memory with BATs as much as possible. BATS can have
any power-of-two size between 128k and 256M but we have only 4 to 8
BATs so the alignment must be good enough to allow efficient use of
the BATs and avoid falling back on standard page mapping which would
kill performance.

So let's go one step forward and make it the default but still allow
users to unset it when wanted.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/057c40164084bfc7d77c0b2ff78d95dbf6a2a21b.1632503622.git.christophe.leroy@csgroup.eu

arch/powerpc/Kconfig

@@ -149,6 +149,7 @@ config PPC
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
 	select ARCH_OPTIONAL_KERNEL_RWX		if ARCH_HAS_STRICT_KERNEL_RWX
+	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
 	select ARCH_STACKWALK
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC	if PPC_BOOK3S || PPC_8xx || 40x