gccNGPackages: write down which monolithic patches are missing

None of `gcc/patches/default.nix` is applied in this set, and it was not
recorded anywhere whether that is deliberate. Go through all of them and say
so, with the reason, so that the next person does not have to derive it
again.

Four are real gaps rather than things this set does not need: `no-sys-dirs`
for native builds, `mangle-NIX_STORE-in-__FILE__` because `cc-wrapper` leaves
`__FILE__` to the compiler for GNU, the `cfi_startproc` reordering on aarch64
-- which belongs to `libgcc` here, not `gcc` -- and `ppc-musl`, which
`no-sys-dirs` does not subsume because `rs6000/sysv4.h` tests
`LOCAL_INCLUDE_DIR` before the `#undef` in `cppdefault.cc` is reached.

One is left open on purpose: `libssp-noshared-musl32` assumes the libc ships
`libssp_nonshared.a`, and this set builds its own `libssp`, which is worth
settling against a real musl x86_32 target rather than on paper.

No patch is added here; this is only the reasoning.

Assisted-by: Claude Code (Claude Opus 5)

pkgs/development/compilers/gcc/ng/README.md

@@ -64,3 +64,23 @@ Both are packaged from the same sources and, for now, the same version: `gccNGPa
 Nothing selects GGN NG yet by default.
 The plan is for very exotic package sets to switch to this first.
 The main tier-1 native Linux package sets cached on `cache.nixos.org` will come later.
+
+### Patches the monolithic set has and this one does not
+
+None of `../patches/default.nix` is applied here.
+Some of it this set does not need, some of it is a real gap; the ones that have been looked at:
+
+| patch | verdict | why |
+|---|---|---|
+| `gcc-12-no-sys-dirs.patch`, `13/no-sys-dirs-riscv.patch` | needed | Needed for a native build, not a cross one. `cppdefault.cc` drops `LOCAL_INCLUDE_DIR` and `NATIVE_SYSTEM_HEADER_DIR` by itself when `CROSS_DIRECTORY_STRUCTURE` is defined and no sysroot is, and `/lib/` and `/usr/lib/` reach `startfile_prefixes` only under `*cross_compile == '0' \|\| target_system_root`. We pass no sysroot, so a cross compiler is already clean and a native one searches all four. |
+| `13/mangle-NIX_STORE-in-__FILE__.patch` | needed | `cc-wrapper` sets `useMacroPrefixMap = !isGNU`, and this set's `gcc` is `isGNU`, so nothing else keeps store paths out of `__FILE__`. |
+| `cfi_startproc-reorder-label-14-1.diff` | needed, in `libgcc` | Needed on aarch64, and belongs to `libgcc` rather than `gcc`: it patches `libgcc/config/aarch64/lse.S`, whose output clang 18 and later refuse to assemble. |
+| `libstdc-fix-compilation-in-freestanding-win32.patch` | believed unnecessary | Only takes effect under `!_GLIBCXX_HOSTED`, and `libstdcxx` is built against a real libc here. The monolithic set needs it because its `withoutTargetLibc` stage builds a freestanding libstdc++. |
+| the Darwin set (`iains` and Homebrew) | needed | Darwin is in scope; it is just not tested yet. Two of the three are `libgcc` patches rather than `gcc` ones, as `cfi_startproc` was: `libgcc-darwin-fix-reexport` (`libgcc/config/t-slibgcc-darwin`) and `libgcc-darwin-detection` (`libgcc/config.host`). Only the large `gcc-16-darwin-aarch64-support` diff belongs to `gcc`. |
+| `c++tools-dont-check-enable-default-pie.patch` | needed | `--enable-default-pie` is about the target, but `c++tools` is built for the host, so it should follow `--enable-host-pie` instead. We pass `--enable-default-pie` and we do build `c++tools` -- there is a `g++-mapper-server` in the output -- so the mis-scoped flag applies here too. Upstream only in 14 and 15, and 15 is the default here, so this does not go away soon. |
+| `ppc-musl.patch` | needed | Needed on powerpc+musl, and `no-sys-dirs` does not subsume it: that one `#undef`s `LOCAL_INCLUDE_DIR` in `cppdefault.cc`, but `rs6000/sysv4.h` tests the macro earlier, where it builds `INCLUDE_DEFAULTS_MUSL_LOCAL`, so `/usr/local/include` survives there. |
+| `gcc-12-gfortran-driving.patch` | needed | Needed wherever libtool parses `gfortran -v`, which `libgfortran` does. Gate it on `langFortran` as the monolithic set does. |
+| the two Cygwin patches | needed, but elsewhere | Neither is upstream in 15 or 16. They belong with whatever enables Cygwin here rather than with this set's own bootstrap. |
+| `libssp-noshared-musl32.patch` | undecided | Makes `LINK_SSP_SPEC` link `-lssp_nonshared` unconditionally, which is an Alpine assumption about what the libc ships, and this set builds its own `libssp`. Worth settling against a real musl x86_32 target rather than on paper. |
+
+All four apply cleanly to 15.3.0 and 16.2.0; none is applied here yet.