Summary:
Currently we use `TestBigEndian` in CMake to determine endianness. This
doesn't work on all platforms and is deprecated since CMake 3.20.
Instead of using CMake, we can just use the GNU/Clang preprocessor
definitions.
The only difficulty is MSVC, mostly because they don't support the same
macros. But, as far as I'm aware, MSVC / Windows targets are always
little endian, and if not we can just override it for that specific
target in the future.
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