The targets specify the soft-float ABI already and -Csoft-float should likely
have never existed in the first place. It is unsound to mix crates compiled
with different float ABI's and the only stable way to get all crates to agree
on the soft float ABI is by specifying a target which uses the soft float ABI
anyway even without explicit -Csoft-float.

See also https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/.60-Csoft-float.60.20considered.20harmful