Release tar files have incorrect file extension

[osteffenrh]

Some toolchain tarballs from the Releases Download section have incorrect file extensions.

They claim to be .tar.xz files, but in fact these were compressed with gzip instead and should carry the .tar.gz extension.

For example https://github.com/loongson/build-tools/releases/download/2022.09.06/loongarch64-clfs-6.3-cross-tools-gcc-full.tar.xz:

Extracting it as xz files does not work:

$ tar xJf ./loongarch64-clfs-6.3-cross-tools-gcc-full.tar.xz                                               
xz: (stdin): File format not recognized
tar: Child returned status 1
tar: Error is not recoverable: exiting now

Here we see that this is in fact a gzip file:

$ file ./loongarch64-clfs-6.3-cross-tools-gcc-full.tar.xz                                                  
./loongarch64-clfs-6.3-cross-tools-gcc-full.tar.xz: gzip compressed data, from Unix, original size modulo 2^32 3332720640

Extracting with the options for gzip works:

$ tar xzvf ./loongarch64-clfs-6.3-cross-tools-gcc-full.tar.xz                                            
cross-tools/
cross-tools/share/
cross-tools/share/locale/
cross-tools/share/locale/da/
cross-tools/share/locale/da/LC_MESSAGES/
[...]