x86emul/test: introduce eq()

x86emul/test: introduce eq()

In preparation for sensible to-boolean conversion on AVX512, wrap
another abstraction function around the present to_bool(<x> == <y>), to
get rid of the open-coded == (which will get in the way of using
built-in functions instead). For the future AVX512 use scalar operands
can't be used then anymore: Use (vec_t){} when the operand is zero,
and broadcast (if available) otherwise (assume pre-AVX512 when broadcast
is not available, in which case a plain scalar is still fine).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>