RFR: 8282475: SafeFetch should not rely on existence of Thread::current [v2]

[Johannes Bechberger]

> The WXMode for the current thread (on MacOS aarch64) is currently stored in the thread class which is unnecessary as the WXMode is bound to the current OS thread, not the current instance of the thread class.
> This pull request moves the storage of the current WXMode into a thread local global variable in `os` and changes all related code. SafeFetch depended on the existence of a thread object only because of the WXMode. This pull request therefore removes the dependency, making SafeFetch usable in more contexts.

Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:

  Minor fixes

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7727/files
  - new: https://git.openjdk.java.net/jdk/pull/7727/files/3890c1e0..886a9354

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7727&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7727&range=00-01

  Stats: 15 lines in 4 files changed: 3 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7727.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7727/head:pull/7727

PR: https://git.openjdk.java.net/jdk/pull/7727