RFR (S) 8232100: GC timings should use proper units for heap sizes

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8232100

Webrev:
  https://cr.openjdk.java.net/~shade/8232100/webrev.01/

GC log prints heap sizes in selected GC events. Currently, it unconditionally uses "M" as the suffix
for heap sizes, which makes GC logs too coarse on smaller heaps. This loses performance data
accuracy, which is sometimes a dealbreaker in logs analysis. Let's make it into proper units.

I ran many tests of my own, but would appreciate if somebody runs it through more comprehensive
suite of tests, looking for tests that parse the GC logs for whatever reason.

Testing: eyeballing GC logs, jdk-submit, hotspot_gc {g1, shenandoah, parallel}

-- 
Thanks,
-Aleksey