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 one
or more of the `queue`, `head`, `tail`, and `last` variables when
compiling with LTO enabled. This commit suggests adding volatile to
these variables to prevent the compiler from tampering with them.

test/cctest/test_node_postmortem_metadata.cc

@@ -172,11 +172,13 @@ TEST_F(DebugSymbolsTest, ReqWrapList) {
   const Argv argv;
   Env env{handle_scope, argv};
 
-  auto queue = reinterpret_cast<uintptr_t>((*env)->req_wrap_queue());
-  auto head =
+  volatile uintptr_t queue =
+      reinterpret_cast<uintptr_t>((*env)->req_wrap_queue());
+  volatile uintptr_t head =
       queue +
       nodedbg_offset_Environment_ReqWrapQueue__head___ListNode_ReqWrapQueue;
-  auto tail = head + nodedbg_offset_ListNode_ReqWrap__prev___uintptr_t;
+  volatile uintptr_t tail = head +
+      nodedbg_offset_ListNode_ReqWrap__prev___uintptr_t;
   tail = *reinterpret_cast<uintptr_t*>(tail);
 
   auto obj_template = v8::FunctionTemplate::New(isolate_);
@@ -194,11 +196,12 @@ TEST_F(DebugSymbolsTest, ReqWrapList) {
   // ARM64 CI machinies.
   for (auto it : *(*env)->req_wrap_queue()) (void) &it;
 
-  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);
-  auto calculated =
+  volatile uintptr_t expected = reinterpret_cast<uintptr_t>(&obj);
+  volatile uintptr_t calculated =
       last - nodedbg_offset_ReqWrap__req_wrap_queue___ListNode_ReqWrapQueue;
   EXPECT_EQ(expected, calculated);