RFR: 8366145: Help diagnose ubsan failure

[Leo Korinth]

This pull request is created to help diagnose the bug report https://bugs.openjdk.org/browse/JDK-8354430.

The problem is that we get a ubsan runtime error when dividing by zero. Ubsan is not normally running, so I will add an assert making it easier to catch the problem and printing the value of the two variables that when subtracted will create a zero; it will hopefully help with diagnosis.

Some comments: 
1) the result of most_recent_gc_end_time_sec() will not change within the function, thus I will clutter the code with a new variable, is that okay?
2) I added two calls to `fabs` which are most probably not needed (ieee floating point rules seems to say that positive and negative zero equals). However I think my proposed code makes it clear that I have thought about it. 

I am now running tier1-3

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Commit messages:
 - 8366145: Help diagnose ubsan failure

Changes: https://git.openjdk.org/jdk/pull/26939/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26939&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366145
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26939.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26939/head:pull/26939

PR: https://git.openjdk.org/jdk/pull/26939