Perform the function selection once, and only if CONFIG_AVX512_OPT
is enabled.  Centralize the selection to xbzrle.c, instead of
spreading the init across 3 files.

Remove xbzrle-bench.c.  The benefit of being able to benchmark
the different implementations is less important than not peeking
into the internals of the implementation.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>