FixedDoubleArrays are a special case:

1 The reads are 64-bit and unaligned, thus use memcpy underneath.
2 The compiler only reads FDArray values for (constant) boilerplate
  elements.

1) makes proper atomic reads tricky-to-impossible without a lock.
Luckily, 2) means we know that the array values are immutable after
initialization, thus we can simply do a non-atomic read from the
compiler thread.

Bug: v8:7790
Change-Id: I39698d867543ce2214a2148511c5d90ced6364b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2848410
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74226}