RFR: 8265702: ZGC on macOS/aarch64 [v2]

[Per Liden]

> This patch enables ZGC on macOS/aarch64. It does three things:
> 1) Enables building of ZGC on this platform.
> 2) Adds `os::processor_id()`, which for now always returns 0.
> 3) Fixes a WX issue in `OptoRuntime::handle_exception_C()`, where the stackwater mark might unnecessarily process a frame when the thread is in WXExec mode. In this case, the we're not touching any oops, so we don't need to process any frames.
> 
> Testing: Passes the same tests as macOS/x86_64 (with the exception of pre-existing issues unrelated to ZGC).

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Add comment to #else

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3609/files
  - new: https://git.openjdk.java.net/jdk/pull/3609/files/c8913936..dd8c42ff

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3609&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3609&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3609.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3609/head:pull/3609

PR: https://git.openjdk.java.net/jdk/pull/3609