Fix sincosf(PI) (#229)

Looks like the implementation was not ported correctly. Some negations
were forgotten in a certain branch. Here is the original code in musl
that has the negations:

https://github.com/bpowers/musl/blob/94cb2ec2a0ffcb47d24dbf7a30e462505396cf54/src/math/sincosf.c#L66-L67

Resolves rust-lang/libm#228

Author: CryZe

libm/src/math/sincosf.rs

@@ -65,11 +65,11 @@ pub fn sincosf(x: f32) -> (f32, f32) {
         /* -sin(x+c) is not correct if x+c could be 0: -0 vs +0 */
         else {
             if sign {
-                s = k_sinf((x + S2PIO2) as f64);
-                c = k_cosf((x + S2PIO2) as f64);
+                s = -k_sinf((x + S2PIO2) as f64);
+                c = -k_cosf((x + S2PIO2) as f64);
             } else {
-                s = k_sinf((x - S2PIO2) as f64);
-                c = k_cosf((x - S2PIO2) as f64);
+                s = -k_sinf((x - S2PIO2) as f64);
+                c = -k_cosf((x - S2PIO2) as f64);
             }
         }
 
@@ -121,3 +121,16 @@ pub fn sincosf(x: f32) -> (f32, f32) {
         _ => (0.0, 1.0),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::sincosf;
+    use crate::_eqf;
+
+    #[test]
+    fn with_pi() {
+        let (s, c) = sincosf(core::f32::consts::PI);
+        _eqf(s.abs(), 0.0).unwrap();
+        _eqf(c, -1.0).unwrap();
+    }
+}