Do not use the already existing always_allocate() method for allowing
all allocations after tear down was started. Use explicit checks
of gc_state() == TEAR_DOWN instead.

This ensures that background threads extend the heap beyond its limits
only when tearing down the isolate. always_allocate() is also used
during regular execution in C++ code.

Bug: v8:10315
Change-Id: I66b5bfb06fa854048f37305a5d0cb7d60fadb30f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390764
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69699}