This new testing allocator for ArrayBuffers uses a small real allocation
that is repeatedly mapped into the requested allocation size. Its purpose
is to allow testing of huge TypedArrays without actually consuming a huge
amount of memory, at the expense of correct behavior (elements will alias
each other). It is only supported on Linux for now, and of course off by
default.

Bug: v8:4153
Change-Id: I4917a78b6190dc075dc4614ebe2696e63addc8c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1962270
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65444}