Change machoread.c to use std::vector

Change machoread.c to use std::vector

This changes machoread.c to use std::vector rather than VEC.  This
allows removing some cleanups.

Regression tested by the buildbot, though I don't think anything
actually tests macho reading.

gdb/ChangeLog
2018-03-23  Tom Tromey  <tom@tromey.com>

	* machoread.c (struct oso_el): Add a constructor.  Don't define as
	a typedef.
	(macho_register_oso): Remove.
	(macho_symtab_read): Take a std::vector.
	(oso_el_compare_name): Now a std::sort comparator.
	(macho_symfile_read_all_oso): Take a std::vector.
	(macho_symfile_read): Use std::vector.  Remove cleanups.