arm64 support?

Hello, thank you for open-sourcing dinit as it looks super interesting and it does solve a use case we have that I can explain more in details right after I am able to test dinit :)

Unfortunately I cannot build it on a Mac M1:

root@89bb8fb5fa8c:/go# go install gitlab.com/tozd/dinit/cmd/dinit@v0.2.0
go: downloading gitlab.com/tozd/dinit v0.2.0
go: downloading gitlab.com/tozd/go/errors v0.5.0
go: downloading golang.org/x/sync v0.2.0
go: downloading golang.org/x/sys v0.9.0
go: downloading gitlab.com/tozd/go/pcontrol v0.0.0-20230619223829-fb29b1c081eb
go: downloading github.com/pkg/errors v0.9.1
go: downloading github.com/google/uuid v1.3.0
# gitlab.com/tozd/go/pcontrol
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:519:35: undefined: unix.SYS_DUP2
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:696:24: originalRegs.Rip undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method Rip)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:734:10: newRegs.Rip undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method Rip)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:735:10: newRegs.Rax undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method Rax)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:736:10: newRegs.Rdi undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method Rdi)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:737:10: newRegs.Rsi undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method Rsi)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:738:10: newRegs.Rdx undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method Rdx)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:739:10: newRegs.R10 undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method R10)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:740:10: newRegs.R8 undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method R8)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:741:10: newRegs.R9 undefined (type "golang.org/x/sys/unix".PtraceRegs has no field or method R9)
pkg/mod/gitlab.com/tozd/go/pcontrol@v0.0.0-20230619223829-fb29b1c081eb/pcontrol.go:741:10: too many errors

root@89bb8fb5fa8c:/go# uname -a
Linux 89bb8fb5fa8c 6.4.10-orbstack-00066-ga79fe089e66c #1 SMP Fri Aug 18 01:40:43 UTC 2023 aarch64 GNU/Linux