add aarch64-unknown-openbsd definition

move c_char definition more deeper to make it 'u8' on
aarch64-unknown-openbsd.
diff --git a/src/unix/bsd/netbsdlike/openbsdlike/openbsd/aarch64.rs b/src/unix/bsd/netbsdlike/openbsdlike/openbsd/aarch64.rs
new file mode 100644
index 0000000..6aa9950
--- /dev/null
+++ b/src/unix/bsd/netbsdlike/openbsdlike/openbsd/aarch64.rs
@@ -0,0 +1,3 @@
+pub type c_long = i64;
+pub type c_ulong = u64;
+pub type c_char = u8;