[cppgc][unittests] Only expect guard pages support on 4k platforms.

Arm64 supports 16k and 64k OS pages, in which case the CPPGC doesn't use
guard pages.

Bug: v8:10808
Change-Id: I36efba687c50b348eda62e9f9094b57bd58b55b5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2485494
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70664}

test/unittests/heap/cppgc/page-memory-unittest.cc

@@ -117,9 +117,16 @@ TEST(PageMemoryRegionTest, PlatformUsesGuardPages) {
   v8::base::PageAllocator allocator;
 #if defined(V8_HOST_ARCH_PPC64) && !defined(_AIX)
   EXPECT_FALSE(SupportsCommittingGuardPages(&allocator));
-#else   // !V8_HOST_ARCH_PPC64
+#elif defined(V8_HOST_ARCH_ARM64)
+  if (allocator.CommitPageSize() == 4096) {
+    EXPECT_TRUE(SupportsCommittingGuardPages(&allocator));
+  } else {
+    // Arm64 supports both 16k and 64k OS pages.
+    EXPECT_FALSE(SupportsCommittingGuardPages(&allocator));
+  }
+#else  // Regular case.
   EXPECT_TRUE(SupportsCommittingGuardPages(&allocator));
-#endif  // !V8_HOST_ARCH_PPC64
+#endif
 }
 
 namespace {