[testsuite] [i386] add -msse2 to tests that require it

Without -msse2, an i586-targeting toolchain fails bf16_short_warn.c
because neither type __m128bh nor intrinsic _mm_cvtneps_pbh get
declared.


for  gcc/testsuite/ChangeLog

	* gcc.target/i386/bf16_short_warn.c: Add -msse2.

gcc/testsuite/gcc.target/i386/bf16_short_warn.c

 /* { dg-do compile } */
-/* { dg-options "-O2" } */
+/* { dg-options "-O2 -msse2" } */
 
 #include<immintrin.h>
 typedef struct {