RFR: 8265682: G1: Mutex::_name dangling in HeapRegionRemSet references after JDK-8264146

[Coleen Phillimore]

This change makes the Mutex constructor stdup the name passed in for the few cases that the name is constructed on the stack.  Most names are shorter than 64 characters so it's still a savings.
Tested with tier1.

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Commit messages:
 - 8265682: G1: Mutex::_name dangling in HeapRegionRemSet references after JDK-8264146

Changes: https://git.openjdk.java.net/jdk/pull/3639/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3639&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265682
  Stats: 27 lines in 2 files changed: 26 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3639.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3639/head:pull/3639

PR: https://git.openjdk.java.net/jdk/pull/3639