PEP-739 build-details generation is incorrect when cross-building

[itamaro]

Bug report

Bug description:

When doing cross-builds, the PEP-739 build-details.json gets generated using inconsistent data, some of which based on the build python, and some from the target platform python.

See for example the following gist:
https://gist.github.com/itamaro/8f23ebec2e142fd7480241b76c16c4e0

The gist contains the build-details.json generated for a 3.15 GIL-disabled build, using a 3.15 GIL-enabled build as the build-python.

Concretely:

cd ~/pybuild/python/main-default
./configure --with-pydebug
make -j

cd ~/pybuild/python/main-nogil
./configure --with-pydebug --disable-gil
make -j

# now use the GIL-enabled python to regenerate build-details.json for the nogil build
_PYTHON_PROJECT_BASE=. _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_td_darwin_darwin _PYTHON_SYSCONFIGDATA_PATH=build/lib.macosx-14.5-arm64-3.15 ~/pybuild/python/main-default/python.exe -E Tools/build/generate-build-details.py /tmp/build-details.json

Some examples from the gist:

Incorrect - abi.flags is ["d"] instead of ["t", "d"]:

  "abi": {
    "flags": [
      "d"
    ]
  },

Correct - libpython.static has the correct SOABI, including t:

  "libpython": {
    "static": "/usr/local/lib/libpython3.15td.a"
  },

Incorrect - suffixes.extensions includes extensions for the GIL-enabled ABI:

    "extensions": [
      ".cpython-315d-darwin.so",
      ".cpython-315-darwin.so",
      ".abi3.so",
      ".so"
    ]

Notes

I used a "cross-build" on the same MacOS host here, with GIL enabled vs GIL disabled, because it was the easiest for a full end-to-end repro on a single machine.
The issue is more general though, as I originally observed it when generating build-info.json files for a MacOS-target runtime (both GIL enabled and disabled) using a Linux-GIL-enabled build-python. That setup is more involved, but the mixing is also more apparent, with some fields using "darwin" and other fields using "linux-gnu-x86_64" as the multiarch string.

CPython versions tested on:

CPython main branch, 3.15, 3.14

Operating systems tested on:

macOS, Linux

[Zheaoli]

I think this issue is caused by that we are mixing up the target and build info. I made a PoC patch for abi.flags and suffixes.extensions. I'm not sure this is acceptable

Zheaoli@f7bfb6c

[itamaro]

cc @FFY00 more generally, should build-details be generated strictly from sysconfigdata, so if sysconfigdata is accurate then build-details would also be accurate?