[binutils-gdb] Constify some commands in macrocmd.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 3088cf40a57ec7b5276f192238603684b4608a90 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 3088cf40a57ec7b5276f192238603684b4608a90

Constify some commands in macrocmd.c

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* macrocmd.c (macro_expand_command, macro_expand_once_command)
	(skip_ws, extract_identifier, macro_define_command)
	(macro_undef_command, macro_list_command): Constify.