Fix arm cfg in float add impl

Author: mattico

src/float/add.rs

@@ -184,13 +184,13 @@ add!(__addsf3: f32);
 add!(__adddf3: f64);
 
 // FIXME: Implement these using aliases
-#[cfg(arm)]
+#[cfg(target_arch = "arm")]
 #[cfg_attr(not(test), no_mangle)]
 pub extern fn __aeabi_dadd(a: f64, b: f64) -> f64 {
     __adddf3(a, b)
 }
 
-#[cfg(arm)]
+#[cfg(target_arch = "arm")]
 #[cfg_attr(not(test), no_mangle)]
 pub extern fn __aeabi_dadd(a: f32, b: f32) -> f32 {
     __addsf3(a, b)