9l: without arguments, exit instead of failing to link nothing

[mmnmnnmnmm]

Found in Arch Linux but never reported upstream: https://bugs.archlinux.org/task/55640

% 9l
/bin/ld: /usr/lib/plan9/lib/lib9.a(main.o): in function `main':
(.text.startup+0x5): undefined reference to `p9main'
collect2: error: ld returned 1 exit status
%