lib{c,sys}: expose _getlogin consistently

Historically we exposed _getlogin as a private symbol on a per-arch basis
(except on aarch64 and riscv) for no obvious reason. We now need to
expose it for libc's use so remove the special case from
makesyscalls.lua and expose it in the generated syscalls.map.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44116