RFR: 8343205: CompileBroker::possibly_add_compiler_threads excessively polls available memory [v2]

[Martin Doerr]

> This PR adds a quick check + bail out in order to avoid excessive usage of slow checks. Especially, it avoids querying the available memory so often.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Minor improvements (review feedback).

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21812/files
  - new: https://git.openjdk.org/jdk/pull/21812/files/ea2fa546..c229422b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21812&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21812&range=00-01

  Stats: 12 lines in 2 files changed: 2 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/21812.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21812/head:pull/21812

PR: https://git.openjdk.org/jdk/pull/21812