RFR: JDK-8301749: Tracking malloc pooled memory size [v2]

[Johan Sjölen]

> This PR implements `os::malloc_info` by calling into glibc's `malloc_info` and provides a JCmd for asking the JVM to call `malloc_info` and print out the results. The user can make an informed decision regarding heap trimming by parsing the output of the JCmd. I wanted to avoid parsing the XML inside of the JVM , as this functionality may turn out to be "good enough". A future enhancement would be for the JVM to make this informed decision by parsing the output. Another may be to present this with other data, for example in NMT.
> 
> @tstuefe, I think you'd be interested in this PR as it builds upon your `trim_native_heap` functionality.
> 
> I'm using weak symbols (as per ELF) in order to compile this for both GLibc and Musl.
> 
> The original bug description is this:
> 
>>Memory not allocated via NMT and uncommitted memory contributes to a discrepancy between NMT and RSS.
> It can be hard to distinguish between these cases.
> If the VM can determine the amount of pooled memory by malloc it would help with both determining if a trim_native_memory is needed, or there are some allocations happening outside of NMT.
>>
>>It would be good if the VM can summarize the malloc pooled memory by using e.g. malloc_info(3).
>>
>>We can thus more precise account for RSS value.

Johan Sjölen has updated the pull request incrementally with four additional commits since the last revision:

 - Fix the test
 - Note man section
 - Add test
 - Reimplement malloc_info as requested

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12455/files
  - new: https://git.openjdk.org/jdk/pull/12455/files/d5ff77c4..a209d711

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12455&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12455&range=00-01

  Stats: 104 lines in 5 files changed: 81 ins; 15 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12455.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12455/head:pull/12455

PR: https://git.openjdk.org/jdk/pull/12455