Partially revert Qthreads patch from #26328 (#26468)

Partially reverts a Qthreads patch from #26328 which resulted in a
performance penalty. The patch was meant to fix some performance
regressions, but instead caused more than it helped.

This PR only reverts the changes to the definition of MACHINE_FENCE.

Closes #26440

[Reviewed by @jhh67]

third-party/qthread/README

@@ -101,17 +101,3 @@ index 2ac887ed..2312c954 100644
    atomic_fetch_sub_explicit(&io_worker_count, 1, memory_order_relaxed);
    pthread_exit(NULL);
 ```
-
-```
---- a/include/qthread/qthread.h
-+++ b/include/qthread/qthread.h
-@@ -87,7 +87,7 @@ using std::memory_order_relaxed;
-
- #include "macros.h"
-
--#define MACHINE_FENCE atomic_thread_fence(memory_order_acq_rel);
-+#define MACHINE_FENCE atomic_thread_fence(memory_order_seq_cst);
-
- #if QTHREAD_ASSEMBLY_ARCH == QTHREAD_AMD64
- #define QTHREAD_SWAPS_IMPLY_ACQ_REL_FENCES
-```

third-party/qthread/qthread-src/include/qthread/qthread.h

@@ -87,7 +87,7 @@ using std::memory_order_relaxed;
 
 #include "macros.h"
 
-#define MACHINE_FENCE atomic_thread_fence(memory_order_seq_cst);
+#define MACHINE_FENCE atomic_thread_fence(memory_order_acq_rel);
 
 #if QTHREAD_ASSEMBLY_ARCH == QTHREAD_AMD64
 #define QTHREAD_SWAPS_IMPLY_ACQ_REL_FENCES