[mini] factor out interp into a library (#6419)

[mini] factor out interp into a library (#6419)

* [mini] factor out interp into a library

It enables us to ship the interpreter as an add-on to the runtime.
Embedder can statically link it into their application on demand.

Usage:

configure mono without the interpreter, such as:

```
$ ./autogen.sh --enable-minimal=interpreter ...
$ make; make install
```

run your favorite program with the interpreter and see it fail:

```
$ ./mono/mini/mono-sgen --interpreter ./mono/mini/basic.exe
Mono IL interpreter support is missing
```

despite `--enable-minimal=interpreter`, we can still build the interp
module:

```
% make -C mono/mini libmono-ee-interp-static.la
  CC       interp/libmono_ee_interp_static_la-interp.lo
  CC       interp/libmono_ee_interp_static_la-mintops.lo
  CC       interp/libmono_ee_interp_static_la-transform.lo
  CCLD     libmono-ee-interp-static.la
```

`libmono-ee-interp-static.a` can be statically linked into the target
application.

* [fixup] include libmono-ee-interp reference in runtime unit-tests