support for "simple gc log" export format (gchisto compatible)

[chewiebug]

Support "simple gc log" as export format to create a file having a line per event in the format
GC_TYPE START_SEC DURATION_SEC

Like this the histogram functionality of gchisto can be used for gc logs that otherwise can't be parsed with gchisto.

see Charlie Hunts suggestion in the hotspot-gc-use mailing list and gchisto (direct link to documentation)

[chewiebug]

implemented in this commit