Rollup merge of #150826 - s390x-asm-f16-vector, r=uweigand,tgross35

Add `f16` inline ASM support for s390x

tracking issue: rust-lang/rust#116909
cc rust-lang/rust#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

Author: JonathanBrouwer