Fix some gettext ARI warnings

Fix some gettext ARI warnings

ARI produces this warning for the lines touched in this patch:

  warning: gettext: All messages should be marked up with _.

However, in these cases, the message is not translatable (they are
syscall names).  Adding an extra set of parentheses silences the
warning.

gdb/ChangeLog:

	* common/scoped_mmap.c (mmap_file): Silence ARI warning.
	* dwarf-index-cache.c (create_dir_and_check): Likewise.
	(test_mkdir_recursive): Likewise.
	* dwarf-index-write.c (write_psymtabs_to_index): Likewise.