[llvm][docs] Refresh Arm builtins cross compile guide (#150966)

Someone asked about this on Discord and it was a bit hard to follow. I
found them a config that worked, but the doc was not as much help as it
should have been.

It probably needs some updates for the runtime build era, but for now,
I'm just making it easier to read. I know the basic build can work at
least.

Some aspects of it may be literally wrong now, but I'll check that
later.

* Remove contractions.
* Remove references to the old separate llvm repo layout.
* Remove mentions of cmake versions older than what llvm requires now.
* Make a bunch of things plain text.
* Make a bunch of things code blocks so they are easier to copy and
paste from.

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