This is a "minimal" change to achieve the required goal: seeing that
there is only one place where we need to indicate that memory should
be reserved with MAP_JIT, we can add a value to the Permissions enum
instead of adding a second, orthogonal parameter.
That way we avoid changing public API functions, which makes this CL
easier to undo once we have platform-independent w^x in Wasm.

Bug: chromium:1117591
Change-Id: I6333d69ab29d5900c689f08dcc892a5f1c1159b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2435365
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70379}