Add default benchmark build config

Fix SEGV by backtrace and GC

Suppress a warning

GitHub: fix #3122

It reverts #3126. #3126 fixes the segmentation fault but generates
broken backtrace.

This change fixes the segmentation fault and generates correct
backtrace. The strategy of this change is "generating backtrace while
irep is alive".

/tmp/test.rb:
    def gen
      e0 = nil
      begin
        1.times {
          raise 'foobar'
        }
      rescue => e
        e0 = e
      end
      e0
    end

    e = gen
    GC.start
    gen
    GC.start

    puts e.backtrace.join("\n")

Run:

    % bin/mruby /tmp/test.rb
    /tmp/test.rb:5:in Object.gen
    /home/kou/work/ruby/mruby.kou/mrblib/numeric.rb:77:in Integral#times
    /tmp/test.rb:4:in Object.gen
    /tmp/test.rb:13

FYI:

    % ruby -v /tmp/test.rb
    ruby 2.3.0p0 (2015-12-25) [x86_64-linux-gnu]
    /tmp/test.rb:5:in `block in gen'
    /tmp/test.rb:4:in `times'
    /tmp/test.rb:4:in `gen'
    /tmp/test.rb:13:in `<main>'

This reverts commit bf7719fe, reversing
changes made to 4f4fa0ad.

We should get backtrace while irep is alive.

    mrbgems/mruby-eval/src/eval.c: In function ‘create_proc_from_string’:
    mrbgems/mruby-eval/src/eval.c:152:10: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
         file = "(eval)";
              ^

Null check for irep & initialize loc.lineno

The original code crashed when mrb->backtrace.n grew to 16.

It looks like the logic to reallocate the backtrace was flawed,
based on the wrong variable (loc_raw->i, which, as I have verified,
decreases from 16 to 0 instead of increasing)

I am not sure if this is the correct fix

Avoid Error when Compiling with -std=c99 flag

[255, 255, 255, 255, 255].hash raises "can't convert Float into Integer"

Fix segfault when `eval("__FILE__")` is executed

Fix bug in mruby-test gem (fix #3094)

Add more limitations

  - defined?
  - alias on global variables
  - Operator modification
  - Kernel.binding missing

In an used build path mruby-test wasn't updating mrbtest.c in the
case that the mgem selection was changed. This lead to:
  - a missing reference in case a GEM was removed
  - ignoring all new GEMs added to the build configuration

This fix keeps track of the active gems and demands a rebuild of
mrbtest.c in case that the gem selection changed.

Add toolchain support for OpenWRT

Addition to mruby limitation documentation

Limitations