Rename clock restriction attribute to "noglobal"

ecp5/globals.cc

diff --git a/ecp5/globals.cc b/ecp5/globals.cc
index 724b0fd..bc5c66d 100644
--- a/ecp5/globals.cc
+++ b/ecp5/globals.cc
@@ -435,8 +435,8 @@
         log_info("Promoting globals...\n");
         auto clocks = get_clocks();
         for (auto clock : clocks) {
-            bool is_global = bool_or_default(clock->attrs, ctx->id("ECP5_IS_GLOBAL"), true);
-            if (!is_global)
+            bool is_noglobal = bool_or_default(clock->attrs, ctx->id("noglobal"), false);
+            if (is_noglobal)
                 continue;
             log_info("    promoting clock net %s to global network\n", clock->name.c_str(ctx));
             if (is_ooc) // Don't actually do anything in OOC mode, global routing will be done in the full design