Add optional argument to command "maint prints c-tdesc"

Add optional argument to command "maint prints c-tdesc"

Nowadays, we need two steps to print c files for xml target description,
that is, 1) read xml target description in, update the current tdesc,
2) visit the current tdesc, print the c file.  It is unnecessary to
involve in current tdesc, and some validations in each gdbarch are
performed unnecessarily, which will reject some target descriptions if
they are missing some mandatory feature.

This patch adds an optional argument to "maint print c-tdesc", which
is an XML file target description, so that we can combine the two
steps above into one step, and don't have to involve in global current
tdesc.

gdb:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* NEWS: Mention it.
	* features/Makefile (%.c: %.xml): Pass the xml file name to
	command "maint print c-tdesc".
	* target-descriptions.c (maint_print_c_tdesc_cmd): Get file
	name from 'arg'.

gdb/doc:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* gdb.texinfo (Maintenance Commands): Document optional
	argument of "maint print c-tdesc".