test: fix DebugSymbolsTest.ReqWrapList on PPC64LE

Currently, DebugSymbolsTest.ReqWrapList fails on PPC64LE when Node has
been configured with Link Time Optimization (LTO) and using RHEL 8.5
and gcc:
```console
$ . /opt/rh/gcc-toolset-11/enable
$ export CC='ccache gcc'
$ export CXX='ccache g++'
$ ./configure --enable-lto
$ make -j8 cctest
...
21:52:27 [ RUN      ] DebugSymbolsTest.ReqWrapList
21:52:27 ../test/cctest/test_node_postmortem_metadata.cc:203: Failure
21:52:27 Expected equality of these values:
21:52:27   expected
21:52:27     Which is: 140736537072320
21:52:27   calculated
21:52:27     Which is: 1099680328560
21:52:27 [  FAILED  ] DebugSymbolsTest.ReqWrapList (43 ms)
```

After looking into this is seems that the compiler is tampering with the
`last` variable when compiling with LTO enabled. This commit suggests
adding volatile to this variable to prevent the compiler from tampering
with it.

PR-URL: #44341
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/cctest/test_node_postmortem_metadata.cc

@@ -194,7 +194,8 @@ TEST_F(DebugSymbolsTest, ReqWrapList) {
   // ARM64 CI machinies.
   for (auto it : *(*env)->req_wrap_queue()) (void) ⁢
 
-  auto last = tail + nodedbg_offset_ListNode_ReqWrap__next___uintptr_t;
+  volatile uintptr_t last =
+      tail + nodedbg_offset_ListNode_ReqWrap__next___uintptr_t;
   last = *reinterpret_cast<uintptr_t*>(last);
 
   auto expected = reinterpret_cast<uintptr_t>(&obj);