Allow to toggle YJIT stats collection from runtime

For use cases where you want to collect the metrics
for a specific piece of code (typically a web request)
you can have the stats turned off by default and then
turn them on at runtime before executing the code you care
about.

yjit.rb

@@ -134,6 +134,14 @@ module YJIT
     Primitive.reset_stats_bang
   end
 
+  def self.stats_enabled?
+    Primitive.cexpr! 'rb_yjit_opts.gen_stats ? Qtrue : Qfalse'
+  end
+
+  def self.stats_enabled=(enabled)
+    Primitive.set_stats_enabled(enabled)
+  end
+
   def self.enabled?
     Primitive.cexpr! 'rb_yjit_enabled_p() ? Qtrue : Qfalse'
   end

yjit_iface.c

@@ -808,6 +808,13 @@ reset_stats_bang(rb_execution_context_t *ec, VALUE self)
     return Qnil;
 }
 
+static VALUE
+set_stats_enabled(rb_execution_context_t *ec, VALUE self, VALUE enabled)
+{
+    rb_yjit_opts.gen_stats = RB_TEST(enabled);
+    return enabled;
+}
+
 #include "yjit.rbinc"
 
 #if YJIT_STATS