[wasm][gc] Make pages inaccessible instead of discarding them

On Linux, both APIs uses {madvise} (and {SetPermissions} additionally
does an {mprotect}), on windows they use different APIs. It turns out that
{SetPermissions} with {kNoAccess} uses {VirtualFree} with
{MEM_DECOMMIT}, so the pages will only be "reserved" afterwards.
{DiscardSystemPages} on the other hand uses the {DiscardVirtualMemory}
which tells the OS that the content of that memory can be discarded,
but the pages stay "committed".
Thus {SetPermissions} with {kNoAccess} frees significantly more memory
than using {DiscardSystemPages}.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I3555462a9c692f15ecb3eca7834eb181617d9ef9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1730992Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63029}

src/wasm/wasm-code-manager.cc

@@ -1251,8 +1251,8 @@ void WasmCodeManager::Decommit(base::AddressRegion region) {
   USE(old_committed);
   TRACE_HEAP("Discarding system pages 0x%" PRIxPTR ":0x%" PRIxPTR "\n",
              region.begin(), region.end());
-  CHECK(allocator->DiscardSystemPages(reinterpret_cast<void*>(region.begin()),
-                                      region.size()));
+  CHECK(allocator->SetPermissions(reinterpret_cast<void*>(region.begin()),
+                                  region.size(), PageAllocator::kNoAccess));
 }
 
 void WasmCodeManager::AssignRange(base::AddressRegion region,