Usage of `host_platform` in compiler activation scripts

### Your question:

This issue stems from a discussion had during the v1-migration of `tzdata`: https://github.com/conda-forge/tzdata-feedstock/pull/34

Rattler-build sets the `target_platform` to `noarch` for a `noarch: generic` recipe.
conda-build does not do that.
This leads to the compiler-activation [script](https://github.com/conda-forge/clang-compiler-activation-feedstock/blob/main/recipe/activate-clang.sh)
```bash
if [ "${CONDA_BUILD_STATE:-0}" = "BUILD" ] && [ "${target_platform:-@TARGET_PLATFORM@}" != "@TARGET_PLATFORM@" ]; then
echo "Not activating environment because this compiler is not expected."
else
activate_clang
fi
```
not being run, and therefore the wrong compilers being used.

For `tzdata` we ended up using the workaround of defining
```yaml
build:
noarch: generic
script:
file: build.sh
env: { target_platform: "${{ host_platform }}" }
```

From the discussion the question

> what should be the value of $target_platform for a noarch recipe be?

arose which is to be picked up in this seperate issue.