gcc@13: fix build on Sequoia and Linux arm64

[cho-m]

• 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>?

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[Bo98]

This really should have all the commits - there's other commits that fix incompatibilities with the macOS 15 SDK and Xcode 16: https://github.com/iains/gcc-13-branch/commits/gcc-13-3-darwin/. Though it seems upstream's still missing iains/gcc-14-branch@3f365c6 and iains/gcc-14-branch@848f27a (cc @fxcoudert).

I'm OK applying them ahead of official r2 tag given the status quo doesn't even build on macOS 15 though I think we should patch a complete set of fixes rather than just the minimum to get bootstrap working.