ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user

The existing ARC variants have 2 issues
 - Use ZOL which may not be present in forthcoming architecture
 - Byte loop based vs. generic version which is word loop based

Signed-off-by: Vineet Gupta <vgupta@kernel.org>

arch/arc/Kconfig

@@ -26,6 +26,8 @@ config ARC
 	select GENERIC_PENDING_IRQ if SMP
 	select GENERIC_SCHED_CLOCK
 	select GENERIC_SMP_IDLE_THREAD
+	select GENERIC_STRNCPY_FROM_USER if MMU
+	select GENERIC_STRNLEN_USER if MMU
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARC_MMU_V4