arm64: microbench: Add time limit for each individual test

Besides using separate running times parameter, we add time limit
for loop_test to make sure each test should be done in a certain
time(5 sec here).

Signed-off-by: Jingyi Wang <wangjingyi11@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
[ Initialize total_ns to zero. ]
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>