arch-x86: Fix writing back 32 bit PTEs in the walker.

The page table walker might need to write back page table entries to set
their accessed bits. It was already checking whether the access was 32
or 64 bit when the PTE was retrieved from the incoming packet, but was
not checking the size when it was written back out, causing an assert to
fail when working with 32 bit legacy PTEs.

Change-Id: I7d02241cad20681e6cac0111edf2454335c466fa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55808
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/pagetable_walker.cc

@@ -530,7 +530,10 @@ Walker::WalkerState::stepWalk(PacketPtr &write)
         // value back to memory.
         if (doWrite) {
             write = oldRead;
-            write->setLE<uint64_t>(pte);
+            if (dataSize == 8)
+                write->setLE<uint64_t>(pte);
+            else
+                write->setLE<uint32_t>(pte);
             write->cmd = MemCmd::WriteReq;
         } else {
             write = NULL;