RFR: 8277556: Call ReferenceProcessorPhaseTimes::set_processing_is_mt once

[Albert Mingkun Yang]

Simple change of moving the call site of `set_processing_is_mt`.

Test: hotspot_gc

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Commit messages:
 - set_processing_is_mt

Changes: https://git.openjdk.java.net/jdk/pull/6500/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6500&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277556
  Stats: 6 lines in 1 file changed: 2 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6500.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6500/head:pull/6500

PR: https://git.openjdk.java.net/jdk/pull/6500