RFR: 8280437: Move G1BufferNodeList to gc/shared

[Kim Barrett]

Please review this simple (trivial?) change, renaming G1BufferNodeList to
BufferNodeList and moving its files from gc/g1 to gc/shared.

Testing:
mach5 tier1

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Commit messages:
 - rename G1BufferNodeList

Changes: https://git.openjdk.java.net/jdk/pull/7183/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7183&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280437
  Stats: 185 lines in 9 files changed: 83 ins; 83 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7183.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7183/head:pull/7183

PR: https://git.openjdk.java.net/jdk/pull/7183