[zerocopy] Re-enable big endian aarch64 types

[197g]

When initially released the LLVM semantics did not agree with the intrinsics in the order of lanes and hence they got removed from stable on big endian targets. The fix of these semantics was shipped with Rust 1.87 in March 2025 (rust-lang/rust#136831).

To keep the library working in these previous compiler versions the intrinsics are enabled by their prior condition, a little endian target is detected, or the feature detection cfg of the more recent rust release is present.

Interesting point: the tests were never gated on target_endian = "little" so it seems that CI is not configured for any big endian aarch64 platform.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (61b2407) to head (e84b8d3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3073   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          20       20           
  Lines        6067     6067           
=======================================
  Hits         5573     5573           
  Misses        494      494           

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[197g]

I don't know what it wants me to change in the CI file. Adding the new flag to the matrix is straightforward but the jobs/build_test/matrix/exclude section is odd. I suppose it should copy whatever is defined for no-zerocopy-aarch64-simd-1-59-0?

[jswrenn]

Thanks for this, and sorry for the late reply! Looks great, just a few small nits.

Addressed.