RFR: 8131330: G1CollectedHeap::verify_dirty_young_list fails with assert

[Kim Barrett]

Please review this fix for a recurrence of a problem with certain uses
of memset on SPARC platforms [1][2].

This change adds a helper function, memset_with_concurrent_readers,
for use by G1 and CMS in situations where a memory region is to be
filled while there may be concurrent readers of that region.  This
function is implemented by simply forwarding to memset on most
platforms.  On SPARC, this function avoids calling memset, because
that function may be implemented using block initialization stores,
which lead to the problems described in [1] and [2].

As part of this change we also eliminate the UseMemSetInBOT
experimental command line option, thereby resolving [3].

The SPARC implementation of memset_with_concurrent_readers is written
using inline assembly code rather than a C++ for-loop, because the
compiler may recognize the for-loop and transform it into a call to
memset, undoing our intentional avoidance of memset.  We see exactly
this happening with Solaris Studio 12.3, for example.  It is this
transformation that led to this recurrence of the problem from [1][2].

CR:
https://bugs.openjdk.java.net/browse/JDK-8131330

Webrev:
http://cr.openjdk.java.net/~kbarrett/8131330/webrev.00/

Testing:
jprt
aurora ad hoc GC/Runtime Nightly, quicktests, tonga, jtreg

[1] https://bugs.openjdk.java.net/browse/JDK-8039042
[2] https://bugs.openjdk.java.net/browse/JDK-6948537
[3] https://bugs.openjdk.java.net/browse/JDK-8042930