arm64: microbench: Add test->post() to further process test results

Under certain circumstances, we need to further process microbench
test results, so we add test->post() in the microbench framework,
later patch will use that.

Signed-off-by: Jingyi Wang <wangjingyi11@huawei.com>
[ Drop total_ns from post() input arguments. ]
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>