RFR: 8273550: Replace os::cgc_thread/pgc_thread with os::gc_thread

[Per Liden]

The os thread types `cgc_thread` and `pgc_thread` might have been treated differently at some point in the past, but today they are not. So I suggest we replace those two types with a single `gc_thread` type.

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Commit messages:
 - 8273550: Replace os::cgc_thread/pgc_thread with os::gc_thread

Changes: https://git.openjdk.java.net/jdk/pull/5444/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5444&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273550
  Stats: 14 lines in 5 files changed: 0 ins; 9 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5444.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5444/head:pull/5444

PR: https://git.openjdk.java.net/jdk/pull/5444