Lazily and dynamically create amd64-linux target descriptions

This patch starts to use the generate c feature files to dynamically
create amd64-linux target descriptions.

gdb:

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

	* amd64-linux-tdep.c: Don't include amd64-XXX-linux and
	x32-XXX-linux.c.  Include 64bit-XX.c and x32-XX.c.
	(amd64_linux_read_description): Create target descriptions.
	(_initialize_amd64_linux_tdep): Don't call initialize_tdesc_XXX
	functions.  Add unit tests.
	* features/Makefile (FEATURE_XMLFILES): Append 64bit-XXX.xml and
	x32-core.xml.
	* features/i386/64bit-avx.c: Generated.
	* features/i386/64bit-avx512.c: Generated.
	* features/i386/64bit-core.c: Generated.
	* features/i386/64bit-linux.c: Generated.
	* features/i386/64bit-mpx.c: Generated.
	* features/i386/64bit-pkeys.c: Generated.
	* features/i386/64bit-segments.c: Generated.
	* features/i386/64bit-sse.c: Generated.
	* features/i386/x32-core.c: Generated.
	* target-descriptions.c (maint_print_c_tdesc_cmd): Print feature
	c files for amd64-linux and x32-linux.