[llvm] [MIPS] soft-promote `f16` (PR #203065)

[Folkert de Vries via llvm-commits]

folkertdev wrote:

I've now tested that, in combination with https://github.com/llvm/llvm-project/pull/203390, this PR is able to build and run the rust `std` test suite, which exercises both `f16` and vectors a bunch. The tests all pass under `qemu` as well. 

In the absence of physical hardware, I think that's the best I can do in terms of testing.

https://github.com/llvm/llvm-project/pull/203065