This patch is the first step for fixing regression introduced in
f998ea39 (digest: introduce FFI bindings for xxHash32/64).
We used xxhash library that is shipped with zstd. However it's
possible that user doesn't use bundled zstd. In such cases we
couldn't export xxhash symbols and build failed with following
error:

```
[ 59%] Linking CXX executable tarantool
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xd80): undefined reference to `XXH32'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xd88): undefined reference to `XXH32_copyState'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xd90): undefined reference to `XXH32_digest'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xd98): undefined reference to `XXH32_reset'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xda0): undefined reference to `XXH32_update'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xda8): undefined reference to `XXH64'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xdb0): undefined reference to `XXH64_copyState'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xdb8): undefined reference to `XXH64_digest'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xdc0): undefined reference to `XXH64_reset'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/tarantool.dir/exports.c.o:(.data.rel+0xdc8): undefined reference to `XXH64_update'
collect2: error: ld returned 1 exit status
```

To avoid a problem this patch introduces standalone xxhash library
that will be bundled anyway. It's worth to mention that our
approach is still related to zstd. We use Cyan4973/xxHash that is
used in zstd and passes the same compile flags to it. Single
difference is usage of XXH_NAMESPACE to avoid symbols clashing
with zstd.

Need for #6135