RFR: JDK-8306507: [linux] Print number of memory mappings in error reports [v2]

[Thomas Stuefe]

> The number of active mappings of a crashed JVM is interesting in case of an OOM, but counting mappings is cumbersome, especially when dealing with very large processes. /proc/pid/maps can have millions of lines.
> 
> Let's print out the number of active mappings when hs-err printing.

Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8306507-Print-number-of-memory-mappings-in-error-reports
 - JDK-8306507-Print-number-of-memory-mappings-in-error-reports

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13555/files
  - new: https://git.openjdk.org/jdk/pull/13555/files/ea2268de..c63ec814

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13555&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13555&range=00-01

  Stats: 49785 lines in 846 files changed: 33838 ins; 11471 del; 4476 mod
  Patch: https://git.openjdk.org/jdk/pull/13555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13555/head:pull/13555

PR: https://git.openjdk.org/jdk/pull/13555