`expr` is failing with multibyte chars

It causes https://github.com/coreutils/coreutils/blob/master/tests/misc/expr-multibyte.pl to fail
```
$ ./target/debug/coreutils expr length αbcdef
7
```

GNU:
```
$ expr length αbcdef
6
```
needs to have a different locale compiled like 
```
sudo locale-gen fr_FR.UTF-8
```