Fix#115150 by encoding f32 and f64 correctly for cross-language CFI. I
missed changing the encoding for f32 and f64 when I introduced the
integer normalization option in #105452 as integer normalization does
not include floating point. `f32` and `f64` should be always encoded as
`f` and `d` since they are both FFI safe when their representation are
the same (i.e., IEEE 754) for both the Rust compiler and Clang.
