reenable fp16 compile in old compiler

cmake/checks/cpu_fp16.cpp

@@ -15,12 +15,12 @@ int test()
 #include "arm_neon.h"
 int test()
 {
-    const float src[] = { 0.0f, 1.0f, 2.0f, 3.0f };
-    short dst[4];
-    float32x4_t v_src = vld1q_f32(src);
+    const float src[] = { 0.0f, 0.0f, 0.0f, 0.0f };
+    short dst[8];
+    float32x4_t v_src = *(float32x4_t*)src;
     float16x4_t v_dst = vcvt_f16_f32(v_src);
-    vst1_f16((__fp16*)dst, v_dst);
-    return dst[0] + dst[1] + dst[2] + dst[3];
+    *(float16x4_t*)dst = v_dst;
+    return (int)dst[0];
 }
 #else
 #error "FP16 is not supported"