features/Makefile: Make 'make cfiles' default to generating all C files

features/Makefile: Make 'make cfiles' default to generating all C files

This makes it easier to rebuild all GDB's generated target description
C files.

It also clarifies the comments a bit.  One might think we need a GDB
configured for the particular arquitecture (--target=foo).  But a
build that includes support for the target description is sufficient.
(GDB rejects target descriptions that explicitly specify the
architecture, with an <architecture> element, if the architecture is
unknown.)

Tested that "make clean-cfiles" deletes all .c files under
src/gdb/features/, and that "make cfiles" generates them all without
error, and that diffing the newly generated C files against master
comes out an empty diff.

gdb/
2014-10-01  Pedro Alves  <palves@redhat.com>

	* features/Makefile: Update comments.
	(XMLTOC): List all xml files we build C files from.
	(clean-cfiles): New rule.