Different FixNum/BigNum boundaries as compared to MRI

Hmm, this is a tricky one. It seems that JRuby and MRI have different limits to separate Fixnum and Bignum.

And from the official ruby docs: "A Fixnum holds Integer values that can be represented in a native machine word (minus 1 bit). If any operation on a Fixnum exceeds this range, the value is automatically converted to a Bignum.".

So, MRI uses 0x3fffffff as the biggest FixNum under 32bit environment.
And JRuby always uses 64bit value: jruby -e 'p (0x7fffffffffffffff + 1).class' ---> Bignum.

The differences are easy to see:

1. ruby -e 'p (0x3fffffff + 1).class' ---> Bignum

2. jruby -e 'p (0x3fffffff + 1).class' ---> Fixnum

Also, this is a root cause for one test failure from rubinius specs:
(basically, division by 0.0 behaves differently in case of FixNum or BigNum).

Note: there is also a separate issue in the rubinius spec test, since the
0x3fffffff + 1 value is hardcoded there, which is not correct for 64bit systems, based on the official spec quoted above.

#>bin/mspec -t jruby spec/core/bignum
.................F.E...............................................................

2)
Bignum#div does NOT raise ZeroDivisionError if other is zero and is a Float FAILED
Infinity:
/opt/work/rubinius/./spec/core/bignum/shared/divide.rb:17
/opt/work/rubinius/./spec/core/bignum/shared/divide.rb:17:in `it'
/opt/work/rubinius/spec/mini_rspec.rb:403:in `it'
/opt/work/rubinius/./spec/core/bignum/shared/divide.rb:17:in `it'
/opt/work/rubinius/./spec/core/bignum/shared/divide.rb:17:in `describe'
/opt/work/rubinius/spec/mini_rspec.rb:399:in `describe'
/opt/work/rubinius/./spec/core/bignum/shared/divide.rb:2:in `describe'
/opt/work/rubinius/./spec/core/bignum/shared/divide.rb:2:in `call'
/opt/work/rubinius/./spec/core/bignum/../../spec_helper.rb:154:in `it_behaves_like'
/opt/work/rubinius/./spec/core/bignum/div_spec.rb:6:in `it_behaves_like'
/opt/work/rubinius/./spec/core/bignum/div_spec.rb:5:in `describe'
/opt/work/rubinius/spec/mini_rspec.rb:399:in `describe'
/opt/work/rubinius/./spec/core/bignum/div_spec.rb:5:in `describe'
/opt/work/rubinius/./spec/core/bignum/div_spec.rb:5:in `load'
last_mspec.rb:11:in `load'
last_mspec.rb:1:in `each'
last_mspec.rb:6:in `each'