Use proper libm power method for f64s

For some reason when attempting to raise floats to powers in
scoring we'd converted our `f64`s to `f32`s, called `libm::powf`
(on no-std targets), then converted the result back to an `f64`.
`libm::pow`, however, exists and raises `f64`s to a power natively.

Here we correct this, using the proper `f64`-native `pow`.

While I doubt its related, I went to look if we have any conversion
in LDK itself in response to
lightningdevkit/ldk-node#483 (where
Android binaries failed to load due to a missing `__extenddftf2`.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -1817,7 +1817,7 @@ fn powf64(n: f64, exp: f64) -> f64 {
 }
 #[cfg(not(feature = "std"))]
 fn powf64(n: f64, exp: f64) -> f64 {
-	libm::powf(n as f32, exp as f32) as f64
+	libm::pow(n, exp)
 }
 
 mod bucketed_history {