implement sincosf and sincosl functions; add prototypes

presumably broken gcc may generate calls to these, and it's said that
ffmpeg makes use of sincosf.

src/linux/sincosf.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <math.h>
+
+void sincosf(float t, float *y, float *x)
+{
+	*y = sinf(t);
+	*x = cosf(t);
+}

src/linux/sincosl.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <math.h>
+
+void sincosl(long double t, long double *y, long double *x)
+{
+	*y = sinl(t);
+	*x = cosl(t);
+}