testsuite/ix86: SSE2 is a prereq to _Float16 use

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.