Support `undef` for `mrb_ary_splice()` instead of `[]`

Removed comments mentioning `MRB_WITHOUT_FLOAT`; ref 2e661e82

The comment is for messages when `MRB_INT16` and `MRB_WITHOUT_FLOAT` are
specified at the same time.
The comment itself is no longer needed now that `MRB_INT16` is gone.

When removing elements from an array, it is possible to avoid creating
an empty array.

Before this patch:

```c
mrb_ary_splice(mrb, ary, head, len, mrb_ary_new(mrb));
```

After this patch:

```c
mrb_ary_splice(mrb, ary, head, len, mrb_undef_value());
```

Thank you @shuujii to additional report on #4973

The difference between `mrb_singleton_class` and `mrb_singleton_class_ptr`:

- `mrb_singleton_class_ptr` returns `struct RClass*`.
- `mrb_singleton_class_ptr` returns `NULL` on immediate values where
  `mrb_singleton_class` raises exceptions.

The tranpoline code in 6a0b68f8 was wrong; reverted.

In ISO, those methods should raise `TypeError`, but the spec has been
changed.  The change was discussed in [Feature#12979].

Check the file descriptor with `IO#initialize`; resolve #4966

Fix `MRB_TT_CPTR` object with `MRB_NAN_BOXING`

Support `MRB_DISABLE_STDIO` for mruby-pack; ref #4954

 Support `MRB_DISABLE_STDIO` for mruby-sprintf; ref #4954

Fixed a build error in mruby-config

Previously, if `MRB_NAN_BOXING` is defined, for example,
`mrb_cptr_value()` could not keep an odd address.

If it is `MRB_32BIT`, it can be embedded in `NaN` as it is.

If it is `MRB_64BIT`, some operations are shared with `MRB_WORD_BOXING`.
In this case, the MRB_API function `mrb_nan_boxing_cptr_value()` is
defined.

In order to share the same operation with `mrb_float_to_str()`,
the internal structure is modified.

Changed to understand `#`, `0`, `-`, ` ` and `+`.

Based on src/stdio/vfprintf.c in git://git.musl-libc.org/musl

Based on src/stdio/vfprintf.c in git://git.musl-libc.org/musl

The build error occurred when the "build/XXX/bin" directory did not exist.

Merged `cygwin_filename()` into `filename()`.

Validate windows by Dir testing

To prevent infinite loop on errors; reported by @shuujii

build when directories and files have spaces

Modified the build to quote filenames so that it builds when files have spaces

Fix `mrb_funcall_with_block()` uses more GC arena