Add "maint check xml-descriptions" to test builtin xml target descriptions

Now, GDB is able to dynamically create i386-linux target descriptions
from features, instead of using pre-generated target descriptions.  These
pre-generated target descriptions are no longer used by GDB (note that
they are still used by GDBserver).

This patch add a new maint command "maint check xml-descriptions" to test
dynamically generated tdesc are identical to these generated from xml files.

gdb:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* cli/cli-cmds.c (maintenancechecklist): New variable.
	* gdbcmd.h (maintenancechecklist): Declare it.
	* i386-linux-tdep.c (_initialize_i386_linux_tdep) [GDB_SELF_TEST]:
	Call i386_linux_read_description with different masks.
	* maint.c (maintenance_check_command): New function.
	(_initialize_maint_cmds): Call add_prefix_cmd.
	* target-descriptions.c (tdesc_reg): override operator != and ==.
	(tdesc_type): Likewise.
	(tdesc_feature): Likewise.
	(target_desc): Likewise.
	[GDB_SELF_TEST] (selftests::record_xml_tdesc): New function.
	(maintenance_check_xml_descriptions): New function.
	(_initialize_target_descriptions) Add command "xml-descriptions".
	* target-descriptions.h (selftests::record_xml_tdesc): Declare.

gdb/testsuite:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* gdb.gdb/unittest.exp: Invoke command
	"maintenance check xml-descriptions".

gdb/doc:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* gdb.texinfo (Maintenance Commands): Document command
	"maint check xml-descriptions".