Reimplement Enumerable#cycle 

Fix pattern of infinite loop
And support all specs in https://github.com/ruby/spec/blob/27960d06e0ce92c37f074450f0eab4b0519b118c/core/enumerable/cycle_spec.rb without Enumerable#size

test/assert.rb should not use puts

Add mrb_class_defined_under

Update documentation of fetch

The sentence `Negative values of +index+ count from the end of the array.` can be interpreted that it only holds if a block is given. Clarify it.

Kernel#respond_to? should return true|false only

Android Task GCC support re-added

Android mips/mips64 support

Removed trailing spaces

Fix unexpected behavior with break

Remove needless MRB_API

```
def yie
  yield
end

def bre
  yie {
    1+1
    break
  }
end

p bre #=> display 2, but should be nil
```

ref #3215

If a function (such as mrb_read_irep_file()) is declared without MRB_API
in header file (such as include/mruby/dump.h), implementation of the
function in source file (such as src/load.c) should also defined without
MRB_API.

If MRB_API is mismatch, Visual C++ reports link error with C2375 error
code: https://msdn.microsoft.com/en-us/library/5k6kw95a.aspx