Revert an accidentally sync.

  Revert "[ruby/open3] Removed to load jit_support.rb"

  This reverts commit 67feb78.

test/lib/jit_support.rb

@@ -0,0 +1,25 @@
+require 'rbconfig'
+
+module JITSupport
+  module_function
+
+  def yjit_supported?
+    return @yjit_supported if defined?(@yjit_supported)
+    # nil in mswin
+    @yjit_supported = ![nil, 'no'].include?(RbConfig::CONFIG['YJIT_SUPPORT'])
+  end
+
+  def yjit_force_enabled?
+    "#{RbConfig::CONFIG['CFLAGS']} #{RbConfig::CONFIG['CPPFLAGS']}".match?(/(\A|\s)-D ?YJIT_FORCE_ENABLE\b/)
+  end
+
+  def rjit_supported?
+    return @rjit_supported if defined?(@rjit_supported)
+    # nil in mswin
+    @rjit_supported = ![nil, 'no'].include?(RbConfig::CONFIG['RJIT_SUPPORT'])
+  end
+
+  def rjit_force_enabled?
+    "#{RbConfig::CONFIG['CFLAGS']} #{RbConfig::CONFIG['CPPFLAGS']}".match?(/(\A|\s)-D ?RJIT_FORCE_ENABLE\b/)
+  end
+end