initial OpenBSD support

core/crypto/rand_generic.odin

@@ -1,6 +1,6 @@
 package crypto
 
-when ODIN_OS != .Linux {
+when ODIN_OS != .Linux && ODIN_OS != .OpenBSD {
 	_rand_bytes :: proc (dst: []byte) {
 		unimplemented("crypto: rand_bytes not supported on this OS")
 	}

core/crypto/rand_openbsd.odin

@@ -0,0 +1,12 @@
+package crypto
+
+import "core:c"
+
+foreign import libc "system:c"
+foreign libc {
+	arc4random_buf :: proc "c" (buf: rawptr, nbytes: c.size_t) ---
+}
+
+_rand_bytes :: proc (dst: []byte) {
+	arc4random_buf(raw_data(dst), len(dst))
+}