glibcLocales: re-enable parallel build

This was originally turned off in #245360 because the approach was
non-reproducible, i.e. all N concurrent jobs were racing to update
`locale-archive`.

glibc's localedef allows to separate the build from the install, i.e.
`localedef --no-archive` allows to only perform the build of the locale
which can be parallelized again. After that, a single process adds all
newly built locales into the archive.

This isn't supported by the Makefiles in upstream glibc yet[1], hence
the additional patches.

For me, this reduces the build-time of `pkgs.glibcLocales` from ~7min to
slightly under 1min.

[1] https://sourceware.org/pipermail/libc-alpha/2025-October/170803.html

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