Remove Linux-facing MacOS references

[Rylan12]

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See Homebrew/brew#16224

This PR removes calls to MacOS that could be accessed on Linux by adding on_macos blocks around the relevant patches in gcc@5 and gcc@6.

In the case of lsyncd, I've reworked the xnu resource to be dependent only on the major macOS version. As pointed out by @Bo98 here, bottles for lsyncd are only matched to a major version and there haven't been issues. Based on this, it seems safe to drop the specificity. When there were different versions for minor releases, I went with the most recent. For example, I used the macOS 14.1 xnu version (10002.41.9) for all Sonoma versions.

I marked this as syntax-only since I don't think we need new bottles or anything, but feel free to change that if we still want to test out the formulae more thoroughly.

[MikeMcQuaid]

Great work here, thanks @Rylan12!