RFR: 8288040: x86: Loom: Improve cont/monitor-count helper methods

[Aleksey Shipilev]

When doing x86_32 port, I realized that cont helper methods are sensitive to `rsp`, which makes the usual trick of using a scratch register for `thread` dangerous. We can just lift the entire thread-acquisition business into the methods themselves. Related to that, we can also do the same thing for monitor-count helper methods, thus fully isolating them from the caller code. It would be even better after [JDK-8286957](https://bugs.openjdk.org/browse/JDK-8286957).

This does the shared code change, so that x86_32 port can hopefully continue as late enhancement after RDP1.

Additional testing:
 - [x] Linux x86_64 fastdebug, `jdk_loom hotspot_loom`
 - [x] Linux x86_32 fastdebug, `jdk_loom hotspot_loom`

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Commit messages:
 - Revert unnecessary
 - Add a guard
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/9086/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9086&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288040
  Stats: 165 lines in 8 files changed: 86 ins; 44 del; 35 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9086.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9086/head:pull/9086

PR: https://git.openjdk.java.net/jdk/pull/9086