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

[Per Liden]

On Thu, 9 Sep 2021 13:16:11 GMT, Per Liden <pliden at openjdk.org> wrote:

> 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.

Thanks all for reviewing!

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