[gdb/build] Return gdb::array_view in thread_info_to_thread_handle

[gdb/build] Return gdb::array_view in thread_info_to_thread_handle

In remote_target::thread_info_to_thread_handle we return a copy:
...
gdb::byte_vector
remote_target::thread_info_to_thread_handle (struct thread_info *tp)
{
  remote_thread_info *priv = get_remote_thread_info (tp);
  return priv->thread_handle;
}
...

Fix this by returning a gdb::array_view instead:
...
gdb::array_view<const gdb_byte>
remote_target::thread_info_to_thread_handle (struct thread_info *tp)
...

Tested on x86_64-linux.

This fixes the build when building with -std=c++20.

Approved-By: Pedro Alves <pedro@palves.net>