Same idea as previous patch, but for add_alias_cmd. Remove the overload
that accepts the target command as a string (the target command name),
leaving only the one that takes the cmd_list_element.
gdb/ChangeLog:
* command.h (add_alias_cmd): Accept target as
cmd_list_element. Update callers.
Change-Id: I546311f411e9e7da9302322d6ffad4e6c56df266