Cygwin: pthread: Correct pthread_cleanup macros to avoid potential syntax errors

This commit revises `pthread_cleanup_push` and `pthread_cleanup_pop`
macros to use a `do { ... } while(0)` wrapper, preventing syntax errors
when used in certain contexts. The original code could fail when they
are wrapped within a `do { ... } while(0)`, causing unintended behavior
or compilation issues.

Example of error:

  #include <pthread.h>

  #define pthread_cleanup_push_wrapper(_fn, _arg) do { \
    pthread_cleanup_push(_fn, _arg); \
  } while (0)

  #define pthread_cleanup_pop_wrapper(_execute) do { \
    pthread_cleanup_pop(_execute); \
  } while (0)

  void cleanup_fn (void *arg) {}

  void *thread_func (void *arg)
  {
    pthread_cleanup_push_wrapper(cleanup_fn, NULL);
    pthread_cleanup_pop_wrapper(1);
    return NULL;
  }

  int main (int argc, char **argv) {
    pthread_t thread_id;
    pthread_create(&thread_id, NULL, thread_func, NULL);
  }

This would fail due to unmatched braces in the macro expansion. The new
structure ensures the macro expands correctly in all cases.

Fixes: 007276b30e ("* cygwin.din: Add _pthread_cleanup_push and _pthread_cleanup_pop.")
Signed-off-by: Shaobo Song <shnusongshaobo@gmail.com>

winsup/cygwin/include/pthread.h

@@ -110,10 +110,13 @@ typedef struct _pthread_cleanup_handler
 void _pthread_cleanup_push (__pthread_cleanup_handler *handler);
 void _pthread_cleanup_pop (int execute);
 
-#define pthread_cleanup_push(_fn, _arg) { __pthread_cleanup_handler __cleanup_handler = \
-					 { _fn, _arg, NULL }; \
-					 _pthread_cleanup_push( &__cleanup_handler );
-#define pthread_cleanup_pop(_execute) _pthread_cleanup_pop( _execute ); }
+#define pthread_cleanup_push(_fn, _arg) \
+  do { \
+    __pthread_cleanup_handler __cleanup_handler = { _fn, _arg, NULL }; \
+    _pthread_cleanup_push(&__cleanup_handler)
+#define pthread_cleanup_pop(_execute) \
+    _pthread_cleanup_pop(_execute); \
+  } while (0)
 
 /* Condition variables */
 int pthread_cond_broadcast (pthread_cond_t *);