[RUNTIME] Use weak link for fp16 functions (#1769)

src/runtime/builtin_fp16.cc

@@ -5,17 +5,16 @@
 */
 
 #include <builtin_fp16.h>
+#include <tvm/runtime/c_runtime_api.h>
 
-namespace tvm {
-namespace runtime {
+extern "C" {
 
-extern "C"  uint16_t __gnu_f2h_ieee(float a) {
+TVM_WEAK uint16_t __gnu_f2h_ieee(float a) {
   return __truncXfYf2__<float, uint32_t, 23, uint16_t, uint16_t, 10>(a);
 }
 
-extern "C" float __gnu_h2f_ieee(uint16_t a) {
+TVM_WEAK float __gnu_h2f_ieee(uint16_t a) {
   return __extendXfYf2__<uint16_t, uint16_t, 10, float, uint32_t, 23>(a);
 }
 
-}  // namespace runtime
-}  // namespace tvm
+}