add math linker test!

Author: ladyada

libraries/CI_Tests/examples/test_cmsis_fast_rfft.ino

@@ -0,0 +1,12 @@
+#include <arm_math.h>
+
+arm_rfft_fast_instance_f32 plan;
+
+void setup() {
+  arm_rfft_fast_init_f32(&plan, 256);
+}
+
+void loop() {
+    float in[256] = { 0 }, out[256] = { 0 };
+    arm_rfft_fast_f32(&plan, in, out, 0);
+}