[GDBserver] unit test to i386_tdesc

This patch adds a unit test in GDBserver to test dynamically created
target descriptions equal these pre-generated ones.

gdb/gdbserver:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* linux-x86-tdesc.c: Include selftest.h.
	(i386_tdesc_test): New function.
	(initialize_low_tdesc): Call selftests::register_test.
	* tdesc.h: Include regdef.h.
	(target_desc): Override operator == and !=.

gdb:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* regformats/regdef.h (struct reg): Override operator == and !=.