MIPS: fix "mipsel-linux-ld: decompress.c:undefined reference to `memmove'"

This is _not_ an upstream commit and just for 5.4.y only.

kernel test robot reported a 5.4.y build issue found by randconfig [1]
after backporting commit 89b158635a ("lib/lz4: explicitly support
in-place decompression"") due to "undefined reference to `memmove'".

However, upstream and 5.10 LTS seem fine. After digging further,
I found commit a510b61613 ("MIPS: Add support for ZSTD-compressed
kernels") introduced memmove() occasionally and it has been included
since v5.10.

This partially cherry-picks the memmove() part of commit a510b61613
to fix the reported build regression since we don't need the whole
patch for 5.4 LTS at all.

[1] https://lore.kernel.org/r/202107070120.6dOj1kB7-lkp@intel.com/
Fixes: defcc2b5e5 ("lib/lz4: explicitly support in-place decompression") # 5.4.y
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/mips/boot/compressed/string.c

@@ -5,6 +5,7 @@
  * Very small subset of simple string routines
  */
 
+#include <linux/compiler_attributes.h>
 #include <linux/types.h>
 
 void *memcpy(void *dest, const void *src, size_t n)
@@ -27,3 +28,19 @@ void *memset(void *s, int c, size_t n)
 		ss[i] = c;
 	return s;
 }
+
+void * __weak memmove(void *dest, const void *src, size_t n)
+{
+	unsigned int i;
+	const char *s = src;
+	char *d = dest;
+
+	if ((uintptr_t)dest < (uintptr_t)src) {
+		for (i = 0; i < n; i++)
+			d[i] = s[i];
+	} else {
+		for (i = n; i > 0; i--)
+			d[i - 1] = s[i - 1];
+	}
+	return dest;
+}