syscall: generate readlen/writelen for openbsd libc

Rather than hand rolling readlen and writelen, move it to being generated
via mksyscall.pl, as is done for most other functions.

Updates #36435

Change-Id: I649aed7b182b41c8639686feae25ce19dab812c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/287532


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>