RFR: 8044107 Add Diagnostic Command to list all ClassLoaders

[Staffan Larsen]

This change adds a new Diagnostic Command to list all ClassLoaders and some statistics for each classloader. The command is called “GC.classloader_stats” and the information listed is:

* An id for the ClassLoader. This is the pointer to the Klass of the ClassLoader. The reason for using the Klass* (instead of the oop) is that it is stable across invocations.
* The id of the ClassLoader’s parent ClassLoader.
* The pointer to the ClassLoaderData structure in the VM. This can be useful for debugging.
* The number of classes loaded by the ClassLoader.
* The total size of all allocated metaspace chunks for the ClassLoader.
* The total size of all allocated metaspace blocks for the ClassLoader.

If there are anonymous classes (invokedynamic classes) attributed to the ClassLoader, the following additional information is listed:
* The number of anonymous classes loaded by the ClassLoader.
* The total size of all allocated metaspace chunks for anonymous classes in the ClassLoader.
* The total size of all allocated metaspace blocks for anonymous classes in the ClassLoader.

The information is gathered during a safe point to guarantee that the data structures are consistent. 

I have added a small test and have run this through jprt. A CCC request has been filed.

webrev: http://cr.openjdk.java.net/~sla/8044107/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8044107

Example output:

ClassLoader         Parent              CLData*            Classes   ChunkSz   BlockSz  Type
0x00000007c002d908  0x0000000000000000  0x0000000000000000       0         0         0  sun.misc.Launcher$ExtClassLoader
0x0000000000000000  0x0000000000000000  0x00007fb239c08de0     761   4694016   4241312  <boot classloader>
                                                                37     75776     50928   + invokedynamic classes
0x00000007c0061028  0x00000007c0036878  0x00007fb239c2de60       1      6144      1976  ClassLoaderStatsTest$DummyClassLoader
                                                                 1      2048      1288   + invokedynamic classes
0x00000007c0036878  0x00000007c002d908  0x00007fb239e10fc0       8     88064     31544  sun.misc.Launcher$AppClassLoader
Total = 4                                                      808   4866048   4327048
ChunkSz: Total size of all allocated metaspace chunks
BlockSz: Total size of all allocated metaspace blocks (each chunk has several blocks)


Thanks,
/Staffan