rts: replace ad-hoc MYTASK_USE_TLV with proper CC_SUPPORTS_TLS

This patch replaces the ad-hoc `MYTASK_USE_TLV` with the
`CC_SUPPORTS_TLS` macro. If TLS support is detected by autoconf, then
we should use that for managing `myTask` in the threaded RTS.

(cherry picked from commit f3017dd3)

rts/Task.c

@@ -52,7 +52,7 @@ Mutex all_tasks_mutex;
 // A thread-local-storage key that we can use to get access to the
 // current thread's Task structure.
 #if defined(THREADED_RTS)
-# if defined(MYTASK_USE_TLV)
+# if CC_SUPPORTS_TLS
 __thread Task *my_task;
 # else
 ThreadLocalKey currentTaskKey;
@@ -75,7 +75,7 @@ initTaskManager (void)
         peakWorkerCount = 0;
         tasksInitialized = 1;
 #if defined(THREADED_RTS)
-#if !defined(MYTASK_USE_TLV)
+#if !CC_SUPPORTS_TLS
         newThreadLocalKey(&currentTaskKey);
 #endif
         initMutex(&all_tasks_mutex);
@@ -109,7 +109,7 @@ freeTaskManager (void)
 
 #if defined(THREADED_RTS)
     closeMutex(&all_tasks_mutex);
-#if !defined(MYTASK_USE_TLV)
+#if !CC_SUPPORTS_TLS
     freeThreadLocalKey(&currentTaskKey);
 #endif
 #endif

rts/Task.h

@@ -265,11 +265,7 @@ extern uint32_t peakWorkerCount;
 // A thread-local-storage key that we can use to get access to the
 // current thread's Task structure.
 #if defined(THREADED_RTS)
-#if ((defined(linux_HOST_OS) && \
-     (defined(i386_HOST_ARCH) || defined(x86_64_HOST_ARCH))) || \
-    (defined(mingw32_HOST_OS) && __GNUC__ >= 4 && __GNUC_MINOR__ >= 4)) && \
-    (!defined(CC_LLVM_BACKEND))
-#define MYTASK_USE_TLV
+#if CC_SUPPORTS_TLS
 extern __thread Task *my_task;
 #else
 extern ThreadLocalKey currentTaskKey;
@@ -287,7 +283,7 @@ extern Task *my_task;
 INLINE_HEADER Task *
 myTask (void)
 {
-#if defined(THREADED_RTS) && !defined(MYTASK_USE_TLV)
+#if defined(THREADED_RTS) && !CC_SUPPORTS_TLS
     return (Task*) getThreadLocalVar(&currentTaskKey);
 #else
     return my_task;
@@ -297,7 +293,7 @@ myTask (void)
 INLINE_HEADER void
 setMyTask (Task *task)
 {
-#if defined(THREADED_RTS) && !defined(MYTASK_USE_TLV)
+#if defined(THREADED_RTS) && !CC_SUPPORTS_TLS
     setThreadLocalVar(&currentTaskKey,task);
 #else
     my_task = task;