unix: correct PtraceIoDesc and define PtraceIoDesc per GOARCH on freebsd

CL 179099 introduced ptrace support for freebsd but it seems the
PtraceIoDesc type was not generated by cgo -godefs through mkall.sh
as it contains an uint member. Regenerate the type and adjust
PtraceIoDesc to deal with the type difference on 32-bit vs. 64-bit.

Change-Id: I1ec9662aa8e1b199ebe557387aa27827f23d9d40
Reviewed-on: https://go-review.googlesource.com/c/sys/+/215242
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/ztypes_freebsd_arm.go b/unix/ztypes_freebsd_arm.go
index a1d4aaf..6f79227 100644
--- a/unix/ztypes_freebsd_arm.go
+++ b/unix/ztypes_freebsd_arm.go
@@ -405,7 +405,7 @@
 	Op   int32
 	Offs *byte
 	Addr *byte
-	Len  uint
+	Len  uint32
 }
 
 type Kevent_t struct {