m32r: add memcpy() for CONFIG_KERNEL_GZIP=y

Fix the m32r link error:

    LD      arch/m32r/boot/compressed/vmlinux
  arch/m32r/boot/compressed/misc.o: In function `zlib_updatewindow':
  misc.c:(.text+0x190): undefined reference to `memcpy'
  misc.c:(.text+0x190): relocation truncated to fit: R_M32R_26_PLTREL against undefined symbol `memcpy'
  make[5]: *** [arch/m32r/boot/compressed/vmlinux] Error 1

by adding our own implementation of memcpy().

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>