GcLatencyTest: min/avg/max reporting in nanoseconds

Report min/avg/max in nanoseconds for GC latency test:

  mavericks:hornet penberg$ java -cp tests GcLatencyTest 1 1
  Running 1 mutators and 1 non-mutator for 1 million cycles...
  Thread       min/avg/max (ns)
  No mutation  0/67/7191000
  Mutator 0    0/204/3021000

Signed-off-by: Pekka Enberg <penberg@kernel.org>