Hadrian: running the testsuite with cross-compilers
Hadrian fails to run the testsuite when GHC is built as a cross-compiler.
```bash
>./configure --target=aarch64-linux-gnu
> ./hadrian/build-stack --flavour=quick --bignum=native --docs=none test
| Run Testsuite GetExtraDeps: none
Unknown program "_build/stage1/lib/bin/aarch64-linux-gnu-ghc-iserv"
Build failed.
```
## Expected
- [x] Support running the testsuite as shown above !7850
- [ ] Automatically switch off Haddock tests so that we don't have to pass `--docs=none`
- [ ] Skip tests requiring to *run* built test programs (they won't as we are cross-compiling)
- [ ] Add an option to run built test programs through a user-specified wrapper (`qemu-*`, `nodejs`...)
- [ ] Add an option to specify the `iserv` program to use and its options
issue