gcc: deduplicate version expressions

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Description of changes

This PR combines our 10 copy-pasted-but-slightly-different gcc expressions into a single Nix expression which uses conditionals where necessary. It reduces the size of our GCC expression by 64% -- from over 5,000 lines of Nix code to under 2,000.

Prior to this PR, making any kind of change uniformly across all versions of GCC was either impossible or extremely error-prone.

This (mostly) completes the work begun in #243607

This PR causes no change in eval -- all gcc packages have the same drvPath after it as they did before:

$ nix-instantiate . -A gcc48 -A gcc49 -A gcc6 -A gcc7 -A gcc8 -A gcc9 -A gcc10 -A gcc11 -A gcc12 -A gcc13
/nix/store/1y7jfw7siqlkz6lnjgl20y54q1bm4v9j-gcc-wrapper-4.8.5.drv
/nix/store/sz8pywyyxxdl42l3pd8kj3wyzfwpr1fm-gcc-wrapper-4.9.4.drv
/nix/store/3q7k9c694c8bcdpdiq4q8z64cr7xjv3d-gcc-wrapper-6.5.0.drv
/nix/store/dbdn4fy29bb86wpm7xsbjsvy27fi7byz-gcc-wrapper-7.5.0.drv
/nix/store/qcih34x7kvs3w0njfmc8k7f9i0hl3fnz-gcc-wrapper-8.5.0.drv
/nix/store/w98c0lzlc774sny2f9ff477mwa3zk4f4-gcc-wrapper-9.5.0.drv
/nix/store/cvq5ghz01az3kl5i7bb2nmiia23ayy16-gcc-wrapper-10.5.0.drv
/nix/store/4b3w1l9fx1398w8m1nwq5p89ri519yqw-gcc-wrapper-11.4.0.drv
/nix/store/bcgvf0frnxj0qxhi04f0f13dkd9naxk7-gcc-wrapper-12.3.0.drv
/nix/store/n3k4pmvv9079j91w9b5slr3nicc6fdlw-gcc-wrapper-13.2.0.drv

The patches/default.nix expression is more complicated than necessary because it preserves the order of the patches exactly. A follow-up PR will greatly simplify that file, but that follow-up PR won't preserve drvPaths like this one does. Likewise, the fixing the GLIBC_DYNAMIC_LINKER... part of postPatch will be made massively simpler by not worrying about keeping all the whitespace exactly the same.

Things done

• Fits CONTRIBUTING.md.

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Rebased.

The last merge of staging into master caused a really nasty merge conflict here.

Since this is strictly cleanup (no change after eval), in order to prevent that from happening again I am going to merge this to both master and staging once ofborg goes green. It's much easier to rebase other patches past this one than it is to rebase this one past other patches.

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The patches/default.nix expression is more complicated than necessary because it preserves the order of the patches exactly. A follow-up PR will greatly simplify that file, but that follow-up PR won't preserve drvPaths like this one does.

Found here:

• gcc: patches: reorganize and simplify #250496

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Likewise, the fixing the GLIBC_DYNAMIC_LINKER... part of postPatch will be made massively simpler by not worrying about keeping all the whitespace exactly the same.

Found here:

• gcc: simplify expression by applying workarounds more broadly #250498

[ghost]

The last merge of staging into master caused a really nasty merge conflict here.

Since this is strictly cleanup (no change after eval), in order to prevent that from happening again I am merging this to both master and staging. It's much easier to rebase other patches past this one than it is to rebase this one past other patches.