syscall: regen zsyscall for openbsd/riscv64

syscall: regen zsyscall for openbsd/riscv64

This removes the unused writelen function, which was cleaned up for other
platforms in CL#529035.

Change-Id: I1999dc81276763bdc73d8590c16729447c4e8538
Reviewed-on: https://go-review.googlesource.com/c/go/+/538119
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Joel Sing <joel@sing.id.au>