Rollup merge of #123159 - chrisnc:fix-arm-rm-none-eabihf-features, r=workingjubilee

Fix target-cpu fpu features on Arm R/M-profile

This is achieved by converting `+<fpu>,-d32,{,-fp64}` to `+<fpu>d16{,sp}`.

By using a single additive feature that captures `d16` vs `d32` and `sp` vs
`dp`, we prevent `-<feature>` from overriding `-C target-cpu` at build time.

Remove extraneous `-fp16` from `armv7r` targets, as this is not included in
`vfp3` anyway, but was preventing `fp16` from being enabled by e.g.,
`-C target-cpu=cortex-r7`, which does support `fp16`.

Author: GuillaumeGomez