Omit the `PAD` argument from generated NetBSD syscall methods

[postmodern]

Instead of exposing the PAD argument explicitly in the generated NetBSD syscall methods, a special :pad symbol should be passed in it's place to X86::NetBSD#syscall_macro and X86_64::NetBSD#syscall_macro. The X86::NetBSD#syscall_macro and X86_64::NetBSD#syscall_macro methods should then push a random register or immediate value for the special :pad argument.