Add initial f16 and f128 support to the aarch64 backend

[beetrees]

This PR adds initial support for passing f16 and f128 values around to the aarch64 backend. Support is added for the load, store, bitcast, select, f16const and f128const CLIF instructions, as well as the fp16 target feature.

As a side effect of this PR generalising aarch64 bitcast support, this PR fixes (for aarch64) #6104.

f16/f128 issue: #8312

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This issue or pull request has been labeled: "cranelift", "cranelift:area:aarch64", "cranelift:meta", "fuzzing", "isle", "wasmtime:api", "wasmtime:config"

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[cfallin]

Sorry for the delay on this one -- all looks reasonable to me. Thanks for this work!

[cfallin]

@beetrees this needs a rebase to latest main I think (due to my delay, sorry); happy to merge once it's ready.

[beetrees]

Rebased

[cfallin]

@beetrees it looks like some new lints are being enforced now (CI fail) -- would you mind taking a look?

[beetrees]

I've fixed the Clippy warnings.