GC needs to be able to read a bigint's length while the main thread may
change the length and the sign (bigints are intentionally mutable as
long as they haven't escaped to user code). Since both values are stored
in the same bitfield, we need to make these accesses atomic.

Also change right-trimming to not insert a filler when the object is
in large object space (it makes no sense there).

Bug: v8:8440
Change-Id: I72a1b6f1eda54566d3cfad554dda1a98ddd61975
Reviewed-on: https://chromium-review.googlesource.com/c/1337737
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57576}