Revert "Switch libm to building with clang."

This reverts commit aadc4b2ff03b12142f2c6163266ebdb1df2a6e76.

The patch broke x86.

Change-Id: I0cda903732a685838bbd1877f94e1593a2a32acc
diff --git a/libm/sincos.c b/libm/sincos.c
index 6bd483a..ad75549 100644
--- a/libm/sincos.c
+++ b/libm/sincos.c
@@ -22,21 +22,18 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
+ *
  */
-
 #define _GNU_SOURCE 1
 #include <math.h>
 
-#if !defined(__clang__)
 // Disable sincos optimization for all functions in this file,
 // otherwise gcc would generate infinite calls.
 // Refer to gcc PR46926.
 // -fno-builtin-sin or -fno-builtin-cos can disable sincos optimization,
 // but these two options do not work inside optimize pragma in-file.
 // Thus we just enforce -O0 when compiling this file.
-// clang doesn't implement this optimization anyway, so it doesn't need this.
 #pragma GCC optimize ("O0")
-#endif
 
 void sincos(double x, double* p_sin, double* p_cos) {
   *p_sin = sin(x);