Improve AVX512 testcase

clang 3.4 managed to accept the original test code, only to fail on the actual Skylake asm later

c_check

@@ -203,8 +203,8 @@ $binformat    = bin64  if ($data =~ /BINARY_64/);
 
 $no_avx512= 0;
 if (($architecture eq "x86") || ($architecture eq "x86_64")) {
-    $code = '"vaddps %zmm1, %zmm0, %zmm0"'; 
-    print $tmpf "void main(void){ __asm__ volatile($code); }\n";
+    $code = '"vbroadcastss -4 * 4(%rsi), %zmm2"';
+    print $tmpf "int main(void){ __asm__ volatile($code); }\n";
     $args = " -o $tmpf.o -x c $tmpf";
     my @cmd = ("$compiler_name $args");
     system(@cmd) == 0;