Change tic6x target descriptions

Change tic6x target descriptions

This patch changes tic6x target descriptions to be more flexible.  Rebuild
tic6x-uclinux GDBserver with my x86 g++, and the unit test passes.

gdb:

2017-11-24  Yao Qi  <yao.qi@linaro.org>

	* arch/tic6x.c: New file.
	* arch/tic6x.h: New file.
	* features/Makefile (FEATURE_XMLFILES): Add tic6x-c6xp.xml,
	tic6x-core.xml and tic6x-gp.xml.
	* features/tic6x-c6xp.c: Generated.
	* features/tic6x-core.c: Generated.
	* features/tic6x-gp.c: Generated.
	* target-descriptions.c (maint_print_c_tdesc_cmd): Match
	"tic6x-".

gdb/gdbserver:

2017-11-24  Yao Qi  <yao.qi@linaro.org>

	* configure.srv: Set $srv_regobj for tic6x-linux.
	* linux-tic6x-low.c: Include "arch/tic6x.h" and "tdesc.h".
	(tic6x_read_description): Move some code to tic6x_arch_setup.
	(tic6x_tdesc_test): New function.
	(initialize_low_arch): Call selftests::register_test.