rename cortex-ar references to unified aarch32

[robamu]

Just a little documentation update. Thanks to the amazing work of @thejpster , we have a unified aarch32-rt run-time for both Cortex-A and Cortex-R.

[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[rustbot]

r? @ehuss

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[ehuss]

cc @chrisnc FYI

r? thejpster
@bors delegate=thejpster rollup

[bors]

✌️ @thejpster, you can now approve this pull request!

If @ehuss told you to "r=me" after making some further change, please make that change, then do @bors r=@ehuss

[rustbot]

Failed to set assignee to thejpster: invalid assignee

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