Adding `ruby` to `PREP` causes the following circular dependencies
because `PREP` is used as a prerequisite by some targets required to
build `ruby` target itself.
```
make: Circular .rbconfig.time <- ruby dependency dropped.
make: Circular builtin_binary.inc <- ruby dependency dropped.
make: Circular ext/extinit.c <- ruby dependency dropped.
make: Circular ruby <- ruby dependency dropped.
```

Adding a new Make variable like `EXTPREP` only for exts may be also
reasonable, but it would introduce another complexity into our build
system. `-bundle_loader` doesn't care that link-time and run-time
loader executables are different as long as bound symbols are provided,
so it's ok to resolve from miniruby to simplify our build.