Add cross-platform compilation support to nimporter CLI
### Issue
I've been doing some development with nimpy & nimporter on an M1 Max (ARM64) and have had quite a few obstacles. Currently, I'm using nimporter to compile on demand in an amd64 Docker container to ensure everything works.

If I tried to precompile the nim files using `nimporter build/compile` I would run into an error when running the Docker container as the binaries were built for arm64, but the container is amd64, which looks like this:

```
Traceback (most recent call last):
  File "main.py", line 7, in <module>
    import nimporter, nim_math
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 914, in _find_spec
  File "/usr/local/lib/python3.8/site-packages/nimporter.py", line 1300, in find_spec
    return Nimporter.import_nim_code(fullname, path, library=True)
  File "/usr/local/lib/python3.8/site-packages/nimporter.py", line 957, in import_nim_code
    cls.__validate_spec(spec)
  File "/usr/local/lib/python3.8/site-packages/nimporter.py", line 1025, in __validate_spec
    raise NimporterException(error_message) from import_error
nimporter.NimporterException: Error importing /nim_math/__pycache__/nim_math.so
Error Message:

    /nim_math/__pycache__/nim_math.so: invalid ELF header
```
Which I have assumed from testing is because the platforms do not match.

### Fix
My suggestion is to add a `--cpu` flag onto the build & compile command, mimicking the flag from the nim compile command [found here](https://nim-lang.org/docs/nimc.html)

This issue isn't pressing as it is still possible to run nimporter and numpy, however, runtime compilation tends to be quite slow.
If you would like, I would be able to add this functionality and create a PR to implement it.

For other context, I've been building exclusively in a Docker container for testing purposes and due to arm64 installation issues with nim (I will be attempting to solve these as I go and will provide any updates here for anyone that is struggling with a similar issue)