After https://crrev.com/c/3484317, passing {nullptr} to the
{CodeSpaceWriteScope} won't work any more. Since the tests do not have a
{NativeModule} to pass instead, make them use
{pthread_jit_write_protect_np} directly.

The jump-table assembler tests have dedicated threads for writing and
executing the code, so we just switch once per thread. The icache test
switches between writing and executing, so we use a little struct for
switching.

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

Bug: v8:12644, v8:11974
Change-Id: I116f3ad75454f749cdc4635802a4617ff91548b2
Cq-Include-Trybots: luci.v8.try:v8_mac_arm64_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_mac_arm64_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3487995Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79290}