Manually enable neon for the vita

[pheki]

Followup of #2749, thank you so much for the tip there!

I've based my changes on how it's done in src/cpu/aarch64/mod.rs.

cargo bench doesn't run directly on the vita, but hacking the bench a bit I see more than a 6x improvement on the iteration time for aead::seal_in_place_separate_tag.

One interesting note here is that cfg(target_feature="neon") does seem to work on current nightly (based on the output of rustc +nightly --print cfg --target=armv7-sony-vita-newlibeabihf) and as the vita is nightly only that would probably be enough, but it could have rustc version dependent behavior for other targets so it may not be the best idea.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.71%. Comparing base (767e8b4) to head (cd9ee98).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2791   +/-   ##
=======================================
  Coverage   96.71%   96.71%           
=======================================
  Files         204      204           
  Lines       20361    20361           
  Branches      519      519           
=======================================
  Hits        19692    19692           
  Misses        552      552           
  Partials      117      117           

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[briansmith]

Thanks! Great work!