libgccjit: add RPATH reference on Intel macOS

[carlocab]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

---

Fixes Homebrew/discussions#5364.

Also, update the test to avoid breaking this again.

[carlocab]

Linux is failing because it can't find the zlib headers. Maybe related to the removal of zlib from the Docker container? CC @Bo98

[Bo98]

Looks like this has never been using Homebrew zlib properly then 🙂

Upstream zlib detection doesn't look like it's very good: https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.1.0/config/zlib.m4. So sounds like we have to pass CFLAGS ourselves, unlike every other dependency where we can pass it properly as an option.

[carlocab]

Note: there is an alternative approach at #171822.

[fxcoudert]

Thanks @carlocab

[github-actions]

🤖 An automated task has requested bottles to be published to this PR.