powerpc/uaccess: Don't set KUEP by default on book3s/32

On book3s/32, KUEP is an heavy process as it requires to
set/unset the NX bit in each of the 12 user segments
everytime the kernel is entered/exited from/to user space.

Don't select KUEP by default on book3s/32.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1492bb150c1aaa53d99a604b49992e60ea20cd5f.1586962582.git.christophe.leroy@c-s.fr

arch/powerpc/platforms/Kconfig.cputype

@@ -377,7 +377,7 @@ config PPC_HAVE_KUEP
 config PPC_KUEP
 	bool "Kernel Userspace Execution Prevention"
 	depends on PPC_HAVE_KUEP
-	default y
+	default y if !PPC_BOOK3S_32
 	help
 	  Enable support for Kernel Userspace Execution Prevention (KUEP)