(gdb-invalidate-assembler): Update assembler buffer

(gdb-invalidate-assembler): Update assembler buffer
correctly when frame changes.
(gdb-info-threads-custom, gdb-threads-buffer-name)
(gdb-display-threads-buffer, gdb-frame-threads-buffer)
(gdb-threads-mode-map, gdb-threads-mode, gdb-get-thread-number)
(gdb-threads-select, gdb-threads-mouse-select): New functions and
variable for a buffer that provides a selectable threads list.