mruby-io: Fixing compilation issue under the legacy MinGW environment

Reported by @shuujii.

Should have handled the case `to_a` returns `nil`.

Adding MRB_MINGW32_LEGACY in common.h in order to identify the legacy MinGW environment (i.e. NOT to be confused with MinGW-w64).
For more info about MinGW defined macros, see: https://sourceforge.net/p/predef/wiki/Compilers/

Simplify MSVC detection to `mrb_static_assert`

Merge pull request #5062 from shuujii/use-normal-static_assert-in-mrb_static_assert-as-much-as-possible

Use normal `static_assert` in `mrb_static_assert` as much as possible

Use `struct _stat32` instead of `struct __stat32`

It is described as `struct __stat32` in the MSVC reference manual.
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/stat-functions?view=vs-2019
But it doesn't really exist, so it must use `struct _stat32`.

It also replaces `struct __stat64` with `struct _stat64` to make it look
nicer.

* `_Static_assert` can also be used with `-std=gnu99` on GCC >= 4.6.
* `static_assert` can be used on MSVC.
* `static_assert` can be used even on old G++/Clang++ if
  `__GXX_EXPERIMENTAL_CXX0X__` is defined.

Allow `mrb_static_assert()` to be used outside of functions

The use of `struct` is an idea by @dearblue.

Note that the home brew version of `mrb_static_assert` only works within
the function body.  This reverts commit 8f99689b.

Merge mruby 2.1.2

Extend the `cipush()` and `cipop()` functions

Improve `rake benchmark`

- Use POSIX format instead of GNU extension for `time` command.
  For example FreeBSD's `time(1)` does not have GNU extensions available.
- Sort `benchmark/bm_*.rb`.
  This is because the order of the bar graph cannot be uniquely
  determined depending on the result of `Dir.glob`.

- Remove ` kh_put_prepare` function used only internally
- Remove `n_occupied` member from `kh_` struct

`puts` should print newline when called without arguments.