[PartitionAlloc] Use fewer VMAs by default on Linux systems

Some clients of PartitionAlloc (for instance mksnapshot, part of
Chromium's build through V8, which uses PartitionAlloc but not with
Chromium's config) do not use Chromium's partition_alloc_support.cc to
configure it. As a result, they still get the default behavior which is
creatig a lot of memory regions.

This is shows to cause crashes in certain build configs on some Linux
systems for mksnapshot. Make the new behavior default for clients that
do not use the feature. This is temporary, the feature flag will be
removed once we settle on what to do on all platforms.

With this patch, mksnapshot succeeds with a limit of 8k VMAs (but not
the full Chromium build, as linking requires more). Tested by running:

$, sudo sh -c 'echo 8000 >  /proc/sys/vm/max_map_count

$ ./mksnapshot --turbo_instruction_scheduling --stress-turbo-late-spilling
--target_os=linux --target_arch=x64 --embedded_src gen/v8/embedded.S
--predictable --no-use-ic --builtins-effects-src
gen/v8/src/builtins/builtins-effects.cc --turbo-elide-frames --embedded_variant
Default --random-seed 314159265 --startup_blob snapshot_blob.bin
--native-code-counters --concurrent-builtin-generation
--concurrent-turbofan-max-threads=0 --verify-heap

Bug: 416540976
Change-Id: I38639512d6b697b5d06747646d642fa2ead57dbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6718547
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1484268}