std_detect: Tidying of slice length

[a4lg]

It's nothing more than a small tidying.
But since stdarch moved on to a Josh subtree, we (stdarch contributors) can ask whether pure stdarch changes are still accepted through this repo.

We don't need to put the length of the riscv_hwprobe array into a variable.
This commit removes that variable and gives the length of the output slice to the __riscv_hwprobe directly.

[rustbot]

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[a4lg]

Hmm, CI is failing with a reason unrelated to this change itself.

[folkertdev]

Generally, changes should still be made here if possible. Certainly right now (CI doesn't run in the rust-lang/rust repo), but this has been the policy for e.g. miri which has been a subtree for a while.

I have a possible fix for the issue here rust-lang/rust#143194, but that'll take at least a day to propagate to nightly and fix CI here. (that is of course one of the reasons to make stdarch a subtree: no more such breakage).