gdb: add missing full stops in --help

The descriptions for most options printed by gdb --help end with a full
stop but, before this patch, not the ones for --args and --interpreter.

This makes the line containing --args a bit longer but still not longer
than the previously longest line, that is the one for the --tty option.

gdb/ChangeLog:

	* main.c (print_gdb_help): Add full stops at the end of the
	descriptions for the --args and --interpreter options.