Add `f16` inline ASM support for s390x
tracking issue: https://github.com/rust-lang/rust/issues/116909
cc https://github.com/rust-lang/rust/issues/125398
Support the `f16x8` type in inline assembly. Only with the `nnp-assist` feature are there any instructions that make use of this type. Based on the riscv implementation I now cast to `i16x8` when that feature is not enabled.
As far as I'm aware there are no instructions operating on `f16` scalar values. Should we still add support for using them in inline assembly?
r? @tgross35
cc @uweigand
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