Improve Time#<=> performance

* time.c (wcmp): use internal cmp() function for comparing internal Fixnum value
    in Time objects. On 64-bit machine, Time object might have Fixnum object
    internally by default and cmp() can compare the Fixnum objects directly.

    Time#<=> will be faster around 60% on 64-bit machine.

* time.c (cmp): add optimized path for comparing internal Bignum value by using
    rb_big_cmp() API. On 32-bit machine, Time object might have Bignum object
    internally by default.

    Time#<=> will be faster around 50% on 32-bit machine.

    [ruby-dev:50034] [Bug #13354] [Fix GH-1546]

### Before
             user     system      total        real
Fixnum   1.410000   0.000000   1.410000 (  1.407848)
Bignum   1.550000   0.000000   1.550000 (  1.549145)

### After
             user     system      total        real
Fixnum   0.880000   0.000000   0.880000 (  0.886662)
Bignum   1.050000   0.000000   1.050000 (  1.047994)

### Test code
require 'benchmark'

Benchmark.bmbm do |x|

  x.report "Fixnum" do
    t1 = Time.now
    t2 = Time.now
    10000000.times do
      t1 <=> t2
    end
  end

  x.report "Bignum" do
    t1 = Time.at(2 ** 64)
    t2 = Time.at(2 ** 64 + 1)
    10000000.times do
      t1 <=> t2
    end
  end

end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58828 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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