Fix vcmul microkernel tester

The tester was using `nanf("")` to initialize the output buffer, but this is not a valid `NaN` value for fp16. This CL changes the tester to use `UINT16_C(0x7E00)`, which is a valid `NaN` value for `fp16`.

PiperOrigin-RevId: 634416194

test/vcmul-microkernel-tester.h

@@ -82,7 +82,7 @@ class VCMulMicrokernelTester {
       } else if (inplace_b()) {
         std::copy(b.cbegin(), b.cend(), y.begin());
       } else {
-        std::fill(y.begin(), y.end(), nanf(""));
+        std::fill(y.begin(), y.end(), UINT16_C(0x7E00) /* NaN */);
       }
       const uint16_t* a_data = inplace_a() ? y.data() : a.data();
       const uint16_t* b_data = inplace_b() ? y.data() : b.data();