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

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

With KMEMCHECK=y, KASAN=n we get this build failure:

  arch/x86/platform/efi/efi.c:673:3:    error: implicit declaration of function ‘memcpy’ [-Werror=implicit-function-declaration]
  arch/x86/platform/efi/efi_64.c:139:2: error: implicit declaration of function ‘memcpy’ [-Werror=implicit-function-declaration]
  arch/x86/include/asm/desc.h:121:2:    error: implicit declaration of function ‘memcpy’ [-Werror=implicit-function-declaration]

Don't #undef memcpy if KASAN=n.

Reported-by: Ingo Molnar <mingo@kernel.org>
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt.fleming@intel.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 769a8089c1fd ("x86, efi, kasan: #undef memset/memcpy/memmove per arch")
Link: http://lkml.kernel.org/r/1443544814-20122-1-git-send-email-ryabinin.a.a@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>