add C stub for sqrtl (already implemented in asm on i386 and x86_64)

src/math/sqrtl.c

@@ -0,0 +1,9 @@
+#include <math.h>
+
+long double sqrtl(long double x)
+{
+	/* FIXME: implement sqrtl in C. At least this works for now on
+	 * ARM (which uses ld64), the only arch without sqrtl asm
+	 * that's supported so far. */
+	return sqrt(x);
+}