[gdb/symtab] Use expand_all_symtabs in maint expand-symtabs

[gdb/symtab] Use expand_all_symtabs in maint expand-symtabs

When issuing a command "maint expand-symtabs", maintenance_expand_symtabs is
called with regexp == nullptr, and calls expand_symtabs_matching like so:
...
      objfile->expand_symtabs_matching
       ([&] (const char *filename, bool basenames)
	{
	  /* KISS: Only apply the regexp to the complete file name.  */
	  return (!basenames
		  && (regexp == NULL || re_exec (filename)));
	},
...

To expand all symtabs gdb usually uses expand_all_symtabs (used for -readnow),
but here we try to handle it in the filename_matcher argument.

Make this more similar to how gdb usually works by using expand_all_symtabs.

A previous version of the patch instead used a nullptr filename_matcher for
the regexp == nullptr case.  That approach regressed test-cases
gdb.dwarf2/dwz-unused-pu.exp and gdb.dwarf2/dw2-dummy.exp.

Tested on x86_64-linux.