gh-84461: Add HOSTRUNNER for program to run Python executable (GH-91931)

`HOSTRUNNER` is a program which can be used to run `BUILDPYTHON` for the host platform (for example, `python.js` requires `node`).

Also change depedencies from `build_all` to `all` so that targets which can't build everything (e.g. WASM) can still run `buildbottest` and `pythoninfo`.

cc @tiran