Compile gcc.dg/vect/pr68305.c with -mavx2 for x86 target

Since gcc.dg/vect/pr68305.c is a compile test, we should add -mavx2 for
x86 target.

	* gcc.dg/vect/pr68305.c (dg-additional-options): Add -mavx2
	for x86 target.

From-SVN: r231820

gcc/testsuite/ChangeLog

+2015-12-18  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* gcc.dg/vect/pr68305.c (dg-additional-options): Add -mavx2
+	for x86 target.
+
 2015-12-18  Nathan Sidwell  <nathan@acm.org>
 
 	* c-c++-common/attr-simd-3.c: Requires pthreads.

gcc/testsuite/gcc.dg/vect/pr68305.c

 /* { dg-do compile } */
 /* { dg-additional-options "-O3" } */
-/* { dg-additional-options "-mavx2" { target avx_runtime } } */
+/* { dg-additional-options "-mavx2" { target { i?86-*-* x86_64-*-* } } } */
 
 int a, b;