Generate c for feature instead of tdesc
    
    This patch changes Makefile and command "maint print c-files" so
    that GDB can print c files for features instead target description.
    Previously, we feed GDB a target description xml file, which generate
    c files including multiple features.
    
    With this patch, in Makefile, we wrap each feature xml file, and
    create a temp target description which include only one feature.
    Then, adjust the target description printer for them, and print
    a c function for each given feature, so that we can use these
    c functions later to create target description in a flexible way.
    
    v3:
     - Add comments,
     - Use startswith,
     - Rename CFILES with TDESC_CFILES,
     - Split the code adding an optional argument in "maint print c-files",
    
    gdb:
    
    2017-07-10  Yao Qi  <yao.qi@linaro.org>
    
    	* features/Makefile (CFILES): Rename with TDESC_CFILES.
    	(FEATURE_XMLFILES): New.
    	(FEATURE_CFILES): New.
    	New rules.
    	(clean-cfiles): Remove generated c files.
    	* features/i386/32bit-avx.c: Generated.
     	* features/i386/32bit-avx512.c: Generated.
     	* features/i386/32bit-core.c: Generated.
     	* features/i386/32bit-linux.c: Generated.
     	* features/i386/32bit-mpx.c: Generated.
     	* features/i386/32bit-pkeys.c: Generated.
     	* features/i386/32bit-sse.c: Generated.
     	* target-descriptions.c: Include algorithm.
    	(tdesc_element_visitor): Add method visit_end.
    	(print_c_tdesc): Implement visit_end.
    	(print_c_tdesc:: m_filename_after_features): Move it to
    	protected.
    	(print_c_feature): New class.
    	(maint_print_c_tdesc_cmd): Use print_c_feature if XML file
    	name starts with "i386/32bit-".