[OSSignpost] Update apinotes to allow usage of os_signpost ABI entrypoint

Currently, the `_os_signpost_emit_with_name_impl` function is not available to be called from Swift. This is the main ABI entrypoint for making os_signpost calls. In order to facilitate more efficient calls to os_signpost in future iterations of the Swift os_signpost API, we need to allow calling this function from Swift.

rdar://70015938

Author: Josh Learn

apinotes/os.apinotes

@@ -63,5 +63,5 @@ Functions:
   SwiftPrivate: true
   NullabilityOfRet: O
 - Name: _os_signpost_emit_with_name_impl
-  Availability: nonswift
-  AvailabilityMsg: 'Use os_signpost'
+  SwiftPrivate: true
+  NullabilityOfRet: O