This commit implements #125759.
It removes:
* Blender does not build on big endian systems anymore.
* Support for opening blendfiles written from a big endian system is
removed.
It keeps:
* Support to generate thumbnails from big endian blendfiles.
* BE support in `extern` or `intern` libraries, including Cycles.
* Support to open big endian versions of third party file formats:
- PLY files.
- Some image files (cineon, ...).
Pull Request: https://projects.blender.org/blender/blender/pulls/140138
bc80ef136e