Add target-specific NaN payloads for the missing tier 2 targets
This PR adds target-specific NaN payloads for the remaining tier 2 targets:
- `arm64ec`: This target is a mix of `x86_64` and `aarch64`, meaning as they both have no extra payloads `arm64ec` also has no extra payloads.
- `loongarch64`: Per [LoongArch Reference Manual - Volume 1: Basic Architecture](https://github.com/loongson/LoongArch-Documentation/releases/download/2023.04.20/LoongArch-Vol1-v1.10-EN.pdf) section 3.1.1.3, LoongArch does quieting NaN propagation with the Rust preferred NaN as its default NaN, meaning it has no extra payloads.
- `nvptx64`: Per [PTX ISA documentation](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#floating-point-instructions) section 9.7.3 (and section 9.7.4 for `f16`), all payloads are possible. The documentation explicitly states that `f16` and `f32` operations result in an unspecified NaN payload, while for `f64` it states "NaN payloads are supported" without specifying how or what payload will be generated if there are no input NaNs.
- `powerpc` and `powerpc64`: Per [Power Instruction Set Architecture](https://files.openpower.foundation/s/9izgC5Rogi5Ywmm/download/OPF_PowerISA_v3.1C.pdf) Book I section 4.3.2, PowerPC does quieting NaN propagation with the Rust preferred NaN being generated if no there are no input NaNs, meaning it has no extra payloads.
- `s390x`: Per [IBM z/Architecture Principles of Operation](https://www.vm.ibm.com/library/other/22783213.pdf#page=965) page 9-3, s390x does quieting NaN propagation with the Rust's preferred NaN as its default NaN, meaning it has no extra payloads.
Tracking issue: #128288
cc ``@RalfJung``
``@rustbot`` label +T-lang
Also cc relevant target maintainers of tier 2 targets:
- `arm64ec`: ``@dpaoliello``
- `loongarch64`: ``@heiher`` ``@xiangzhai`` ``@zhaixiaojuan`` ``@xen0n``
- `nvptx64`: ``@RDambrosio016`` ``@kjetilkjeka``
- `powerpc`: the only documented maintainer is ``@BKPepe`` for the tier 3 `powerpc-unknown-linux-muslspe`.
- `powerpc64`: ``@daltenty`` ``@gilamn5tr`` ``@Gelbpunkt`` ``@famfo`` ``@neuschaefer``
- `s390x`: ``@uweigand`` ``@cuviper``
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