Fix column alignment in harness-gc iteration output

[k0kubun]

The marking, sweeping, and maj/min columns were wider than their headers, causing misaligned output.

Example

Before

  itr:   time   marking  sweeping  gc_count     major     minor  maj/min
   #1: 1063ms      68.0ms      34.0ms         3         1         2      0.50
   #2:  845ms       9.0ms       4.0ms         1         0         1      0.00

After

  itr:   time   marking  sweeping  gc_count     major     minor  maj/min
   #1: 1055ms    57.0ms    30.0ms         2         1         1     1.00
   #2:  850ms     8.0ms    12.0ms         1         0         1     0.00