This teaches unix/mksyscall.go about mapping uint64 to syscall
arguments, similar to existing handling of int64.

Change-Id: I536b4b09bbf6a8e3016565a4a04d2b9d32cbc624
Reviewed-on: https://go-review.googlesource.com/c/164664Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>