RFR: 8298725: Add BitMap support for reverse iteration

[Kim Barrett]

Please review this addition of BitMap::reverse_iterate, which is like
BitMap::iterate but visits the set bits in a range from high index to low.

Testing:
mach5 tier1, which includes running the updated gtests on all Oracle-supported
platform

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Commit messages:
 - add reverse_iterate

Changes: https://git.openjdk.org/jdk/pull/13151/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13151&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298725
  Stats: 140 lines in 3 files changed: 116 ins; 13 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/13151.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13151/head:pull/13151

PR: https://git.openjdk.org/jdk/pull/13151