Native aarch64 rebuild breaks cross-aarch64 C++ extension linking downstream (LDCXXSHARED regression)

Downstream cross-aarch64 recipes with a C++ Python extension (e.g. numba-cuda, libnvshmem) started failing at link time after python-feedstock's aarch64 build was flipped from cross to native. The compiler stack and the recipe-side sed are both unchanged; what changed is what CPython's own `configure` step emits into the sysconfigdata.

## When it landed

The "go native for arm builds" change removed `linux_aarch64: linux_64` from `conda-forge.yml`, landing on 2026-06-11 to 2026-06-12 across every currently-live branch except `3.12`:

- `main`: [`babe1dd57b`](https://github.com/conda-forge/python-feedstock/commit/babe1dd57b) in #874
- `3.13`: [`5a74e3a0ab`](https://github.com/conda-forge/python-feedstock/commit/5a74e3a0ab) in #877
- `3.11`: [`e20ad42441`](https://github.com/conda-forge/python-feedstock/commit/e20ad42441) in #875
- `3.10`: [`5ddb2eb7c5`](https://github.com/conda-forge/python-feedstock/commit/5ddb2eb7c5) in #876

Each PR bundled the flip with an unrelated main change (version bump or openssl backport); the flip itself is a one-liner in `conda-forge.yml`.

## Symptom (downstream cross-aarch64 link step)

```
$PREFIX/compiler_compat/ld: $BUILD_PREFIX/bin/../libexec/gcc/aarch64-conda-linux-gnu/14.3.0/liblto_plugin.so: error loading plugin: ... cannot open shared object file: No such file or directory
```

Misleading error — the actual issue is the aarch64 `ld` shim in `$PREFIX/compiler_compat/` being handed an x86_64 LTO plugin path via `--plugin`.

## Bisect (numba-cuda-feedstock main, last-green vs first-fail)

- `gcc_impl_linux-aarch64` / `binutils_impl_linux-aarch64` / `ld_impl_linux-aarch64`: identical (`14.3.0=h77f067d_19` / `2.45.1=default_hc5fa074_102` / `2.45.1=default_h27e1c4c_102`)
- `python` (linux-aarch64): `3.10.20=h28be5d3_0_cpython` → `3.10.20=h4f76b5d_1_cpython`

Extracting `_sysconfigdata_aarch64_conda_linux_gnu.py` (the "raw configure" variant, saved *before* the recipe-side sed) from each package:

- `h28be5d3_0` (cross-built): `'CXX': 'aarch64-conda-linux-gnu-c++'`
- `h4f76b5d_1` (native-built): `'CXX': 'aarch64-conda-linux-gnu-c++ -pthread'`

`LDCXXSHARED = "$(CXX) -shared"`, so it picks up the same `-pthread` on native. Then [`recipe/build_base.sh#L453-L456`](https://github.com/conda-forge/python-feedstock/blob/888e71e2/recipe/build_base.sh#L453-L456):

```bash
sed -i.bak "s@-pthread@-pthread -B $PREFIX/compiler_compat@g" sysconfigfile
```

turns the default sysconfigdata's `LDCXXSHARED` into `'g++ -pthread -B $PREFIX/compiler_compat -shared'`. Setuptools uses `LDCXXSHARED` verbatim for C++ extension links, so `-B $PREFIX/compiler_compat` reaches the cross-toolchain link line and routes `ld` through the aarch64 shim. On a linux-64 build host driving an x86_64 GCC, the aarch64 `ld` can't handle the x86_64 LTO plugin GCC hands it via `--plugin`, and the link fails.

## Why C-only cross-builds haven't been failing the same way

`LDSHARED` has always had `-B $PREFIX/compiler_compat` in it too, but patch [`0019-Fix-LDSHARED-when-CC-is-overriden-on-Linux-too.patch`](https://github.com/conda-forge/python-feedstock/blob/888e71e2/recipe/patches/0019-Fix-LDSHARED-when-CC-is-overriden-on-Linux-too.patch) makes distutils' `customize_compiler` strip the sysconfig `CC` prefix off `LDSHARED` and replace it with `os.environ['CC']`. Since `LDSHARED` starts with exactly the sysconfig `CC` string (`'gcc -pthread -B $PREFIX/compiler_compat'`), the entire `-pthread -B compiler_compat` chunk is consumed along with `gcc`, leaving only `-shared -Wl,...` in the resulting command. No equivalent exists for `LDCXXSHARED`, so it's used as-is.

## Why 3.12 isn't affected

The `3.12` branch of this feedstock still has `linux_aarch64: linux_64` in `conda-forge.yml` — it wasn't included in the "go native" flip. Correspondingly, only `linux_aarch64_python3.12` passes in [numba-cuda-feedstock run 28638905979](https://github.com/conda-forge/numba-cuda-feedstock/actions/runs/28638905979); all other Python versions fail on aarch64.

## Confirmed affected feedstocks

- [libnvshmem-feedstock#35](https://github.com/conda-forge/libnvshmem-feedstock/pull/35) — hit 2026-06-12, worked around with `-fno-use-linker-plugin`
- [numba-cuda-feedstock#92](https://github.com/conda-forge/numba-cuda-feedstock/pull/92) — hit 2026-07-03, same workaround

## Suggested fixes

1. Extend patch `0019` to also strip the sysconfig `CXX` prefix off `LDCXXSHARED` — symmetric with the C side. Preserves the sed's intent for native/system-gcc users.
2. Or scope the sed itself away from `CXX`/`LDCXXSHARED` so `-pthread -B compiler_compat` never enters those vars.

Happy to test either against numba-cuda-feedstock's CI once a rebuild is available.