[Threading] Move stack bounds fetching into the threading library.

Fetching the current stack bounds is done using threading functions, so
should be in the threading library.

rdar://90776105

include/swift/Threading/Impl/Linux.h

@@ -61,6 +61,8 @@ inline bool threads_same(thread_id a, thread_id b) {
   return ::pthread_equal(a, b);
 }
 
+stack_bounds thread_get_current_stack_bounds();
+
 // .. Mutex support ..........................................................
 
 using mutex_handle = ::pthread_mutex_t;