testsuite/ix86: SSE2 is a prereq to _Float16 use

When enabling AVX512FP via attribute or pragma, the _Float16 type would
remain unavailable when at initialization time SSE2 wouldn't be seen as
available for use. While this may hint at a wider underlying issue (like
the feature, the type may want providing dynamically, albeit this may be
challenging in particular for functions returning _Float16 yet having
the attribute specified after their return type), for now simply make
SSE2 available when targeting ix86.

gcc/testsuite/

	* gcc.target/i386/avx512fp16-reduce-op-2.c: Force SSE2 for i?86.
	* gcc.target/i386/pr99464.c: Likewise.

gcc/testsuite/gcc.target/i386/avx512fp16-reduce-op-2.c

 /* { dg-do compile } */
 /* { dg-options "-O2 -mprefer-vector-width=512 -fdump-tree-optimized" } */
+/* { dg-additional-options "-msse2" { target i?86-*-* } } */
 
 /* { dg-final { scan-tree-dump-times "\.REDUC_PLUS" 3 "optimized" } } */
 /* { dg-final { scan-tree-dump-times "\.REDUC_MIN" 3 "optimized" } } */

gcc/testsuite/gcc.target/i386/pr99464.c

 /* PR target/99464 */
 /* { dg-do compile } */
 /* { dg-options "-O2" } */
+/* { dg-additional-options "-msse2" { target i?86-*-* } } */
 
 #pragma GCC target("arch=cannonlake")