Add riscv64 support (#81)

diff --git a/ztypes_riscv64.go b/ztypes_riscv64.go
new file mode 100644
index 0000000..459bc9a
--- /dev/null
+++ b/ztypes_riscv64.go
@@ -0,0 +1,11 @@
+// Code generated by cmd/cgo -godefs; DO NOT EDIT.
+// cgo -godefs types.go
+
+// +build riscv64
+
+package pty
+
+type (
+	_C_int  int32
+	_C_uint uint32
+)