The SFI's flags can be changed by the mutator while the concurrent marker is checking
the flags for bytecode flushing. None of the flag bits checked for bytecode flushing
are changed, however since they are in an int32 field TSAN will complain if any of the
other flags are changed while reading from another flag. Fix this by making the flags
use the RELAXED_INT32_ACCESSORS.

BUG=v8:8592,v8:8395

Change-Id: I5fbb4fd381c2b288abf0cd36eb0b8256e1929af6
Reviewed-on: https://chromium-review.googlesource.com/c/1382458Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58325}