Remove two cleanups using std::string

Remove two cleanups using std::string

This patches removes cleanups from a couple of spots by using
std::string rather than manual memory management.

Regression tested by the buildbot, though note that I don't believe
the buildbot actually exercises the machoread code.

gdb/ChangeLog
2018-03-13  Tom Tromey  <tom@tromey.com>

	* machoread.c (macho_check_dsym): Change filenamep to a
	std::string*.
	(macho_symfile_read): Update.
	* symfile.c (load_command): Use std::string.