[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.