Auto merge of #130987 - thejpster:revise-arm-platform-notes-soft-float, r=ehuss

Revise arm platform notes regarding soft float

This PR updates the Arm microcontroller platform docs to recommend `-fpregs` instead of `+soft-float` as [discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/.60-Ctarget-feature.3D.2Bsoft-float.60.20considered.20harmful)

Author: bors