Rename RubyVM::MJIT to RubyVM::JIT

because the name "MJIT" is an internal code name, it's inconsistent with
--jit while they are related to each other, and I want to discourage future
JIT implementation-specific (e.g. MJIT-specific) APIs by this rename.

[Feature #17490]

test/net/smtp/test_smtp.rb

@@ -31,7 +31,7 @@ def readline
     def setup
       # Avoid hanging at fake_server_start's IO.select on --jit-wait CI like http://ci.rvm.jp/results/trunk-mjit-wait@phosphorus-docker/3302796
       # Unfortunately there's no way to configure read_timeout for Net::SMTP.start.
-      if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled?
+      if defined?(RubyVM::JIT) && RubyVM::JIT.enabled?
         Net::SMTP.prepend Module.new {
           def initialize(*)
             super