[Darwin] Fix cleanup leak in machoread.c:macho_symfile_read

[Darwin] Fix cleanup leak in machoread.c:macho_symfile_read

This patch fixes a cleanup leak in macho_symfile_read (symbol_table):

          symbol_table = (asymbol **) xmalloc (storage_needed);
          make_cleanup (xfree, symbol_table);

Unfortunately, fixing the leak alone triggers a crash which occurs
while loading the symbols from an executable:

    % gdb
    (gdb) file g_exe
    [SIGSEGV]

The crash is caused by the fact that performing the cleanup
right after the call to macho_symtab_read, as currently done,
is too early.

Indeed, references to this symbol_table get saved in the oso_vector
global during the call to macho_symtab_read via calls to
macho_register_oso, and those references then get accessed
later on, when processing all the OSOs that got pushed (see
call to macho_symfile_read_all_oso).

This patch prevents this by using one single cleanup queue for
the entire function, rather than having additional separate
cleanup queues (Eg: for the handling of the minimal symbols),
thus preventing the premature free'ing of the minimal_symbols
array.

Secondly, this patch takes this opportunity for avoiding the use
of the oso_vector global, thus making it simpler to track its
lifetime.

gdb/ChangeLog:

        * machoread.c (oso_vector): Delete this global.
        (macho_register_oso): Add new parameter "oso_vector_ptr".
        Use it instead of the "oso_vector" global.
        (macho_symtab_read, macho_symfile_read_all_oso): Likewise.
        (macho_symfile_read): Use a local oso_vector, to be free'ed
        at the end of this function, in place of the old "oso_vector"
        global.  Update various function calls accordingly.  Use one
        single cleanup chain for the entire function.