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

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 89b158635ad7 ("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 a510b616131f ("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 a510b616131f
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: defcc2b5e54a ("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>