cmake HAVE_GNU_F2H_IEEE examples

pytorch/FBGEMM cmake/modules/FindGnuH2fIeee.cmake :14

CHECK_CXX_SOURCE_COMPILES("
    #include <arm_neon.h>
    int main() {
        float f = 1.0f;
        uint16_t h = __gnu_f2h_ieee(f);
        return 0;
    }
" HAVE_GNU_F2H_IEEE)