fix: detect native c++ stdlib instead of hard-coding for darwin

[jacobly0]

This PR Darwin is not the only target that can have libcxx installed as the default c++ stdlib.

Closes #12658

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8f80881c2f7611c61d0228dac94b27582b44d5d0 --onto e7e3588c973226e85d79ed0f3154cfca79b61c6f. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-23 20:15:20)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 8f80881c2f7611c61d0228dac94b27582b44d5d0 --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-02-23 20:15:22)