util: fix test on msvc

We also miss this function on MSVC. But let's use the functionality in
meson to check for supported functions instead of hand-rolling the list
here.

Fixes: 067023dce2 ("util: Add some unit tests of the half-float conversions.")
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16290>

src/util/tests/half_float_test.cpp

@@ -38,8 +38,7 @@
 #define HALF_NEG_INF 0xfc00
 #define HALF_NAN 0x7e00
 
-#if defined(ANDROID) || defined (__MINGW32__)
-/* Android and mingw were missing this function, roll our own as a best effort. */
+#ifndef HAVE_ISSIGNALING
 static bool issignaling(float x)
 {
    uint32_t ui = fui(x);