Add support for freebsd/arm64 (#87)

ztypes_freebsd_arm64.go

diff --git a/ztypes_freebsd_arm64.go b/ztypes_freebsd_arm64.go
new file mode 100644
index 0000000..4418139
--- /dev/null
+++ b/ztypes_freebsd_arm64.go
@@ -0,0 +1,13 @@
+// Code generated by cmd/cgo -godefs; DO NOT EDIT.
+// cgo -godefs types_freebsd.go
+
+package pty
+
+const (
+	_C_SPECNAMELEN = 0xff
+)
+
+type fiodgnameArg struct {
+	Len int32
+	Buf *byte
+}