RFR (S): 8131734: Add free_archive_regions support to G1 for -Xshared:auto

[Tom Benson]

Hi,
Please review this change for JDK-8131734, an addition to the Archive 
Region support in G1 to handle -Xshared:auto behavior.  With 
Xshared:auto, if the shared string region file mapping fails, sharing is 
disabled and execution continues.  To allow this, the archive region 
space which has already been allocated needs to be freed.   A 
free_archive_regions() entry is added to g1CollectedHeap, to be called 
if the file mapping performed by CDS fails.  Its arguments are the same 
as those given to alloc_archive_regions (and that would have been given 
to fill_archive_regions if the mapping had succeeded).

The CDS code change will be posted separately by Jiangli Zhou.

JBS: https://bugs.openjdk.java.net/browse/JDK-8131734
Webrev: http://cr.openjdk.java.net/~tbenson/8131734/webrev/
Tested: JPRT, and benchmarks run with alloc/free_archive_regions calls 
forced at init time and heap verification enabled.
Jiangli also tried this with the failing Xshared:auto test and the 
corresponding CDS code change to call free_archive_regions.

Thanks,
Tom