expect PlatformUsesGuardPages to return true on AIX

AIX default page size is 4096 bytes hence
PlatformUsesGuardPages returns true. Power Linux however
has a default page size of 65536 bytes and the above function
is expected to return false. More info is available
at https://crrev.com/c/2144060.

Change-Id: I35a13ada5bd1b18729cfa039a0bc699a409fbc2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2147634Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#67134}

test/unittests/heap/cppgc/page-memory_unittest.cc

@@ -116,7 +116,7 @@ TEST(PageMemoryRegionTest, PlatformUsesGuardPages) {
   // This tests that the testing allocator actually uses protected guard
   // regions.
   v8::base::PageAllocator allocator;
-#ifdef V8_HOST_ARCH_PPC64
+#if defined(V8_HOST_ARCH_PPC64) && !defined(_AIX)
   EXPECT_FALSE(SupportsCommittingGuardPages(&allocator));
 #else   // !V8_HOST_ARCH_PPC64
   EXPECT_TRUE(SupportsCommittingGuardPages(&allocator));