[ruby/benchmark] Adjust ljust Benchmark#bm with labels was not using the highest length among the labels to adjust the correct ljust. Instead of printing the result during the report generation, now it is waiting to print the result once it is generated.

Benchmark.bm { |x|
  x.item("aaaa") { 1 }
  x.item("aaaaaaaa") { 0 }
}

After

              user     system      total        real
aaaa      0.000005   0.000002   0.000007 (  0.000003)
aaaaaaaa  0.000001   0.000001   0.000002 (  0.000002)

Before

       user     system      total        real
aaaa  0.000005   0.000001   0.000006 (  0.000003)
aaaaaaaa  0.000002   0.000001   0.000003 (  0.000003)

https://github.com/ruby/benchmark/commit/3e74533ead