Get --target, etc., from sysconfig when cross compiling

[benfogle]

I am the maintainer of crossenv, and since cryptography started using setuptools-rust, I've started receiving issues since cross compiling that package no longer works out of the box. This can be fixed by gathering more information from sysconfig and passing that along to rustc.

(PR incoming)

[jameshilliard]

Not sure if you're running into the exact same issue here but I'm seeing a potentially related issue when cross compiling with buildroot.

[messense]

It is indeed error-prone when invoking bdist_wheel with --plat-name.

https://github.com/messense/manylinux2014-cross-arm/runs/2199462499 here I've asked it to build manylinux2014_aarch64 wheel and it does build the .so file with aarch64 gcc but named it wrong to x86_64.

❯ file pyo3_test.cpython-36m-x86_64-linux-gnu.so
pyo3_test.cpython-36m-x86_64-linux-gnu.so: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, with debug_info, not stripped

[benfogle]

I think that issue is common to all Python. bdist_wheel only uses --plat-name to generate the output file name, so I think you'd see this no matter what module you were trying to compile. (The other commands aren't much better: build_ext, for example, tells you that --plat-name is for cross-compiling, but it only checks it if you're running Windows.)