Move DWARF-constant stringifying code to new file

Move DWARF-constant stringifying code to new file

This moves the DWARF debugging functions that stringify various
constants to a new file, dwarf2/stringify.c.

gdb/ChangeLog
2020-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf_unit_type_name, dwarf_tag_name)
	(dwarf_attr_name, dwarf_form_name, dwarf_bool_name)
	(dwarf_type_encoding_name): Move to stringify.c.
	* Makefile.in (COMMON_SFILES): Add dwarf2/stringify.c.
	* dwarf2/stringify.c: New file.
	* dwarf2/stringify.h: New file.