Skip fast path for dec2flt when optimize_for_size
Tracking issue: https://github.com/rust-lang/rust/issues/125612
Skip the fast algorithm when optimizing for size.
When compiling for https://github.com/quartiq/stabilizer I get these numbers:
Before
```
text data bss dec hex filename
192192 8 49424 241624 3afd8 dual-iir
```
After
```
text data bss dec hex filename
191632 8 49424 241064 3ada8 dual-iir
```
This saves 560 bytes.
fe1dc02163