pthread_jit_write_protect* functions are only available on arm64 Mac,
not on iOS (which also sets V8_{TARGET_,}OS_MACOSX).
This CL refactors the logic to detect whether pthread_jit_write_protect
and MAP_JIT are available and defines a global preprocessor macro which
can subsequently be used instead of the existing complex condition.

R=jkummerow@chromium.org, mlippautz@chromium.org

Change-Id: I63894f42df35406d6eee90a4ce5070c2fde7b566
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3077154Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Auto-Submit: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76143}