ARM: Fix !kuser helpers case

Fix yet another build failure caused by a weird set of configuration
settings:

  LD      init/built-in.o
arch/arm/kernel/built-in.o: In function `__dabt_usr':
/home/tom3q/kernel/arch/arm/kernel/entry-armv.S:377: undefined reference to `kuser_cmpxchg64_fixup'
arch/arm/kernel/built-in.o: In function `__irq_usr':
/home/tom3q/kernel/arch/arm/kernel/entry-armv.S:387: undefined reference to `kuser_cmpxchg64_fixup'

caused by:
CONFIG_KUSER_HELPERS=n
CONFIG_CPU_32v6K=n
CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG=n
Reported-by: NTomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -357,7 +357,8 @@ ENDPROC(__pabt_svc)
 	.endm
 
 	.macro	kuser_cmpxchg_check
-#if !defined(CONFIG_CPU_32v6K) && !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
+#if !defined(CONFIG_CPU_32v6K) && defined(CONFIG_KUSER_HELPERS) && \
+    !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
 #ifndef CONFIG_MMU
 #warning "NPTL on non MMU needs fixing"
 #else