cpu::arm: Use `cfg_if!` to clarify target conditionals.

[briansmith]

Clarify the conditions in which each implementation is used. Move each OS-specific implementation into its own source file.

To review:

git difftool HEAD^1:src/cpu/arm.rs src/cpu/arm/darwin.rs
git difftool HEAD^1:src/cpu/arm.rs src/cpu/arm/linux.rs
git difftool HEAD^1:src/cpu/arm.rs src/cpu/arm/fuchsia.rs
git difftool HEAD^1:src/cpu/arm.rs src/cpu/arm/windows.rs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.28%. Comparing base (c610b14) to head (fe3c19a). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2040 +/- ## ======================================= Coverage 96.28% 96.28% ======================================= Files 141 143 +2 Lines 20459 20459 Branches 226 226 ======================================= Hits 19698 19698 Misses 727 727 Partials 34 34 ```

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