Change stabilization version of no_std from 1.0 to 1.6.

src/libsyntax/feature_gate.rs

@@ -307,7 +307,7 @@ declare_features! (
     (accepted, issue_5723_bootstrap, "1.0.0", None),
     (accepted, macro_rules, "1.0.0", None),
     // Allows using #![no_std]
-    (accepted, no_std, "1.0.0", None),
+    (accepted, no_std, "1.6.0", None),
     (accepted, slicing_syntax, "1.0.0", None),
     (accepted, struct_variant, "1.0.0", None),
     // These are used to test this portion of the compiler, they don't actually