pico_float_none is not supported on Risc-V

[will-v-pi]

When calling pico_set_float_implementation(${TARGET} none), such as done by pico_minimize_runtime, it adds -msoft-float which is an Arm-only compile option

I think the fix is to wrap

pico-sdk/src/rp2_common/pico_float/CMakeLists.txt

Line 110 in a1438df

target_compile_options(pico_float_none INTERFACE -msoft-float)

in if (NOT PICO_RISCV) so it doesn't get added for Risc-V binaries, but I'm not sure if there's a different option that should be passed instead?