std: Move the cmath module into the sys module

This commit moves the `f32::cmath` and `f64::cmath` modules into the
`sys` module. Note that these are not publicly exported modules, simply
implementation details. These modules are already platform-specific with
shims on MSVC and this is mostly just a reflection of that reality. This
should also help cut down on `#[cfg]` traffic if platforms are brought on
which don't directly support these functions.

src/libstd/sys/windows/mod.rs

@@ -25,6 +25,7 @@ pub mod args;
 #[cfg(feature = "backtrace")]
 pub mod backtrace;
 pub mod c;
+pub mod cmath;
 pub mod condvar;
 #[cfg(feature = "backtrace")]
 pub mod dynamic_lib;