Fix prototype of getlogin_r syscall to match userland prototype (use size_t

Fix prototype of getlogin_r syscall to match userland prototype (use size_t
for the length argument).

Fortunately, this only affects 64-bit platforms and all of those would pass
the argument in registers, so there is no BE64 regression to expect and no
need to renumber that system call.

Found with afresh1@ as part of his perl syscall emulator.