Reland "cppgc: Enable pointer compression by default on Desktop"

- The data race on atomic memcpying/memsetting was fixed;
- All the known alignment issues in Blink were fixed;
- Several perf optimizations were applied.

Original change's description:
> cppgc: Enable pointer compression by default on Desktop
>
> The CL enables pointer compression in Oilpan.
>
> For sherrifs: the CL may cause some slight perf regressions (likely
> blink_perf.*), due to slightly higher cost of compression and
> decomrpession.
>
> Speedometer2 is not expected to regress, as was checked locally. Such a
> slight performance degradation is compensated by memory savings that are
> expected to be around 10-20% of Oilpan committed size (~2.5-5% of Renderer
> PMF).

Bug: chromium:1325007
Change-Id: I5fa9a06cb1fa5141f4e2b22e710007e2404a176b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3762567
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81914}

BUILD.gn

@@ -826,6 +826,12 @@ if (cppgc_enable_caged_heap) {
 
   # Always enable young generation compile time flag if caged heap is enabled.
   cppgc_enable_young_generation = true
+
+  if (is_linux || is_mac || is_win) {
+    # Always enable on desktop but avoid on ChromeOS for now (for stability
+    # reasons).
+    cppgc_enable_pointer_compression = true
+  }
 }
 if (cppgc_enable_young_generation) {
   enabled_external_cppgc_defines += [ "CPPGC_YOUNG_GENERATION" ]