Move code to new file dwarf2/macro.c

Move code to new file dwarf2/macro.c

This moves some more code out of dwarf2/read.c, introducing new files
dwarf2/macro.c and dwarf2/macro.h.

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

	* dwarf2/read.c (dwarf2_macro_malformed_definition_complaint)
	(macro_start_file, consume_improper_spaces)
	(parse_macro_definition, skip_form_bytes, skip_unknown_opcode)
	(dwarf_parse_macro_header, dwarf_decode_macro_bytes)
	(dwarf_decode_macros): Move to macro.c.
	* dwarf2/macro.c: New file.
	* dwarf2/macro.h: New file.
	* Makefile.in (COMMON_SFILES): Add dwarf2/macro.c.