RFR: 8265702: ZGC on macOS/aarch64

[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).

-------------

Commit messages:
 - 8265702: ZGC on macOS/aarch64

Changes: https://git.openjdk.java.net/jdk/pull/3609/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3609&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265702
  Stats: 13 lines in 3 files changed: 7 ins; 1 del; 5 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