Ensure that GetContentHash produces the same value on different endian machines

[CyrusNajmabadi]

Fixes #74682

1. We don't have a good way to test htis, absent a dedicated test run on a big endian machine (which is what the runtime team does).
2. We can definitely take out that copy of the vectorized code, and say if you're before .net 8 that you get the slow impl.