configure: add configure option avx512f_opt

If it is enabled, config-host.mak will have CONFIG_AVX512F_OPT defined.

AVX512F instruction set is available since Intel Skylake, and can be enabled in
compiling with -mavx512f.
More info:
https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf



Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>