Enable FPU for RP235X Core1

[Totto8492]

Fix #3353

[Dirbaio]

We should do like cortex-m-rt for consistency: enable the FPU only when building for -eabihf targets.

(I think there are reasons to not enable the FPU if you don't need it? like lower power consumption?)

https://github.com/rust-embedded/cortex-m/blob/master/cortex-m-rt/build.rs#L69-L71
https://github.com/rust-embedded/cortex-m/blob/master/cortex-m-rt/src/lib.rs#L597

[Totto8492]

Oh I see. And maybe instead, should be able to easily activate the FPU only when needed.