workaround gcc bug 46926 by providing a dumb sincos implementation

note that this library itself is built with -ffreestanding so sincos.c
should not be miscompiled even if the gcc used to compile musl has
this bug.

src/linux/sincos.c

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