core/crypto: Add has_rand_bytes

This allows runtime detection as to if `rand_bytes` is supported or not,
and lets us enable the test-case on all of the supported targets.

core/crypto/crypto.odin

@@ -1,3 +1,7 @@
+/*
+package crypto implements a selection of cryptography algorithms and useful
+helper routines.
+*/
 package crypto
 
 import "core:mem"
@@ -51,3 +55,9 @@ rand_bytes :: proc (dst: []byte) {
 
 	_rand_bytes(dst)
 }
+
+// has_rand_bytes returns true iff the target has support for accessing the
+// system entropty source.
+has_rand_bytes :: proc () -> bool {
+	return _has_rand_bytes()
+}