Do nothing when empty string

Fix #3361

This issue was reported by https://hackerone.com/mg36

Added example for cross-compiling on Renesas RX630

Reported by https://hackerone.com/haquaman.

Reported by Denis Kasak https://hackerone.com/dkasak

Fix crash when exponent is -2147483648

Check type before calling mrb_range_ptr.

Reported from @clayton-shopify.

Also msym size changed to 512 from 256.

Support pkg-config in mrbgem.rake

Implement Object#freeze

Example usage:

    MRuby::Gem::Specification.new('mruby-onig-regexp') do |spec|
      # ...
      if spec.search_package('onigmo')
        # Use onigmo.h when onigmo.pc exist.
        spec.cc.defines += ["HAVE_ONIGMO_H"]
      elsif spec.search_package('oniguruma')
        # Use oniguruma.h when oniguruma.pc exist.
        spec.cc.defines += ["HAVE_ONIGURUMA_H"]
      else
        # Use bundled Onigmo otherwise.
        # ...
      end
    end

Mark all the built-in classes during GC sweep

Don't generate code for NODE_NEGATE if the result isn't used
Reported by https://hackerone.com/haquaman

do not destroy a page with an active TT_ENV

Fix segfault in gen_values with NOVAL and more than 127 args

Fix segfault when undef is called with more than 126 arguments