Fix gdb_xml_debug/gdb_xml_error ATTRIBUTE_PRINTF use

Fix gdb_xml_debug/gdb_xml_error ATTRIBUTE_PRINTF use

The declarations of gdb_xml_debug and gdb_xml_error are passing "0" as
"first-to-check" argument to ATTRIBUTE_PRINTF, as if they were va_args
functions.  Consequently, the arguments to gdb_xml_debug /
gdb_xml_error aren't being checked against the format strings.

With that fixed, a couple obvious bugs are exposed, both fixed by this
commit.

gdb/ChangeLog:
2017-03-27  Pedro Alves  <palves@redhat.com>

	* xml-support.h (gdb_xml_debug): Pass a "first-to-check" argument
	to ATTRIBUTE_PRINTF.
	* solib-target.c (library_list_start_list): Print "string" not
	"version".
	* xml-tdesc.c (tdesc_start_field): Pass "field_name" to
	gdb_xml_error call.