x86, kasan: Fix build failure on KASAN=y && KMEMCHECK=y kernels

Declaration of memcpy() is hidden under #ifndef CONFIG_KMEMCHECK.
In asm/efi.h under #ifdef CONFIG_KASAN we #undef memcpy(), due to
which the following happens:

  In file included from arch/x86/kernel/setup.c:96:0:
  ./arch/x86/include/asm/desc.h: In function ‘native_write_idt_entry’:
  ./arch/x86/include/asm/desc.h:122:2: error: implicit declaration of function ‘memcpy’ [-Werror=implicit-function-declaration]   memcpy(&idt[entry], gate, sizeof(*gate));
    ^
    cc1: some warnings being treated as errors
    make[2]: *** [arch/x86/kernel/setup.o] Error 1

We will get rid of that #undef in asm/efi.h eventually.
But in the meanwhile move memcpy() declaration out of #ifdefs
to fix the build.
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1444994933-28328-1-git-send-email-aryabinin@virtuozzo.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/string_64.h

@@ -27,12 +27,11 @@ static __always_inline void *__inline_memcpy(void *to, const void *from, size_t
    function. */
 
 #define __HAVE_ARCH_MEMCPY 1
+extern void *memcpy(void *to, const void *from, size_t len);
 extern void *__memcpy(void *to, const void *from, size_t len);
 
 #ifndef CONFIG_KMEMCHECK
-#if (__GNUC__ == 4 && __GNUC_MINOR__ >= 3) || __GNUC__ > 4
-extern void *memcpy(void *to, const void *from, size_t len);
-#else
+#if (__GNUC__ == 4 && __GNUC_MINOR__ < 3) || __GNUC__ < 4
 #define memcpy(dst, src, len)					\
 ({								\
 	size_t __len = (len);					\