GDB perf test on disassemble

This patch adds a test case to test the performance of GDB doing
disassembly.

gdb/testsuite/

2013-11-28  Yao Qi  <yao@codesourcery.com>

	* lib/gdb.exp (with_gdb_prompt): New proc.
	* gdb.perf/disassemble.exp: New.
	* gdb.perf/disassemble.py: New.