Improved `Class#new` method

Fix failed build with `MRB_NAN_BOXING` and `enable_cxx_abi`

For `MRB_NAN_BOXING` and `MRB_WORD_BOXING`.

```console
% c++ -xc++ -std=c++03 -S -Iinclude -DMRB_NAN_BOXING -DMRB_NO_PRESYM -o- src/array.c > /dev/null
In file included from src/array.c:7:
In file included from include/mruby.h:115:
In file included from include/mruby/value.h:201:
include/mruby/boxing_nan.h:95:12: error: cannot initialize return object of type 'enum mrb_vtype' with an rvalue of type 'int'
    return (enum mrb_vtype)(o.u >> 8) & 0x1f;
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

Avoid redefining `MRB_WORDBOX_NO_FLOAT_TRUNCATE`

Avoid violations of the `mrb_static_assert()` standard

Integers out of 32 bit range will be allocated in the heap.

Favor pointer means encode NaN boxed values to keep pointer values
unmodified, to reduce the cost of far frequent pointer value retrievals.

ref: #5564

```console
% cc -pedantic -S -Iinclude -DMRB_NO_PRESYM -o- src/array.c > /dev/null
In file included from src/array.c:7:
In file included from include/mruby.h:115:
In file included from include/mruby/value.h:204:
include/mruby/boxing_word.h:133:1: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]
mrb_static_assert(sizeof(mrb_value) == sizeof(union mrb_value_));
^
include/mruby.h:109:108: note: expanded from macro 'mrb_static_assert'
    mrb_static_assert_expand(mrb_static_assert_selector(__VA_ARGS__, mrb_static_assert2, mrb_static_assert1)(__VA_ARGS__))
                                                                                                           ^
include/mruby.h:100:10: note: macro 'mrb_static_assert_selector' defined here
# define mrb_static_assert_selector(a, b, name, ...) name
         ^
1 warning generated.
```

Adding `MRB_WORDBOX_NO_FLOAT_TRUNCATE` to the build configuration in 32-bit CPU mode had a double definition.

```console
% cat myconf.rb
MRuby::Build.new do
  toolchain "clang"
  defines << "MRB_WORDBOX_NO_FLOAT_TRUNCATE"
  cc.flags << "-m32"
  linker.flags << "-m32"
  enable_debug
end

% rake CONFIG=myconf.rb
CPP   src/array.c -> build/host/src/array.pi
In file included from /var/tmp/mruby/src/array.c:7:
In file included from /var/tmp/mruby/include/mruby.h:115:
In file included from /var/tmp/mruby/include/mruby/value.h:203:
/var/tmp/mruby/include/mruby/boxing_word.h:11:10: warning:
      'MRB_WORDBOX_NO_FLOAT_TRUNCATE' macro redefined [-Wmacro-redefined]
# define MRB_WORDBOX_NO_FLOAT_TRUNCATE
         ^
<command line>:3:9: note: previous definition is here
#define MRB_WORDBOX_NO_FLOAT_TRUNCATE 1
        ^
1 warning generated.
...SNIP...
```

The number of registers used is reduced.

Also, previously `R6` and` R7` were used, which exceeded the limit of `new_irep.nregs = 6`.
This could cause the VM stack to overrun.

Preparation for:

* 64 bit integer with NaN Boxing
* Favor pointer NaN Boxing

WIP code was mistakenly committed.

Make `mrb_static_assert()` a variable argument

`mrb_static_assert()` extends the macro function to take one or two arguments.
If the argument is other than that, an error will occur.

References:
- static_assert のメッセージ省略を許可 - cpprefjp C++日本語リファレンス
  https://cpprefjp.github.io/lang/cpp17/extending_static_assert.html
- c - Overloading Macro on Number of Arguments - Stack Overflow
  https://stackoverflow.com/a/11763277

Added testing support for cross-MinGW builds.

This adds a build_config that will cross-build a Windows executable
using the MinGW cross-compiler and will also run the unit (i.e.
'rake test') using Wine.

For this to work, I made some modifications to the underlying test
scripts as well as some minor changes to a couple of the tests
themselves.

Since we have added several new instructions.

These instructions call methods of the receiver.

* `R(a)` -> `R[a]`
* `Pool(a)` -> `Pool[a]`
* `Syms(a)` -> `Syms[a]`
* `Irep(a)` -> `Irep[a]`

build(deps): bump actions/checkout from 2.3.4 to 2.3.5

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.3.4 to 2.3.5.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2.3.4...v2.3.5)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Fix a syntax error in commented-out code

By uncommenting the line changed by this commit, `ruby -c build_config/default.rb` complains of a syntax error due to the illegally nested double quotes

1024 was too big.  Some deep recursion could not be detected before
actual stack overflow.

Since `s2` is `NULL` on top-level, it will be an error. No need to
generate strict bytecode.

Fix grammar in doc/guides/mrbgems.md

Need to add `n` because the stack will be shifted in the `mrb_funcall`.