Summary

Keep the sysconfig generator aware of compiler paths that appeared in older downloadable python-build-standalone releases, so regenerating the mappings on current main does not drop support for previously shipped aarch64 and riscv64 binaries.

Context

uv patches compiler paths embedded in managed Python sysconfig data to use portable cc and c++ commands. These mappings are generated from the current python-build-standalone targets.

Older python-build-standalone releases remain downloadable and retain the compiler paths used when they were built. When a target changes toolchains or is removed from the current targets, regenerating the mappings silently drops support for those releases.

This previously removed the aarch64 GNU mappings. The upcoming python-build-standalone sync in #19531 would similarly replace the riscv64 GNU mappings with LLVM mappings.

Test plan

I've manually verified, that before this patch, on an arm64 linux box:

% uvx python3.12.10 -c 'import sysconfig; print(sysconfig.get_config_var("CC"))'
/usr/bin/aarch64-linux-gnu-gcc -pthread

and after (after uninstalling the python version using uv python uninstall 3.12.10):

% ./target/debug/uvx python3.12.10 -c 'import sysconfig; print(sysconfig.get_config_var("CC"))'
cc -pthread