Update arm vcvt intrinsics to use llvm.fpto(su)i.sat

[JamieCunliffe]

Those intrinsics have the correct semantics for the desired fcvtz instruction,
without any undefined behaviour. The previous simd_cast was undefined for
infinite and NaN which could cause issues.

[rust-highfive]

r? @Amanieu

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[bjorn3]

I think simd_cast should be changed instead, probably keeping the old behavior as simd_cast_unchecked.

[Amanieu]

I'll defer the simd_cast question to @rust-lang/project-portable-simd. For now this is good enough to fix stdarch.