Fix unfinished changes

Absolute path for windows

Enable MRB_METHOD_T_STRUCT by default on 32bit Windows

Because we can't use the highest 2 bits of function pointers.

Improving comments in the cross-compiling sample for the Sega Dreamcast.

Add CIFuzz action

Added example for cross-compiling for Sega Dreamcast (sh-elf).

Updated README.md - Fixed link to github issues

Thanks to @yujiyokoo for demonstrating mruby usage on Sega Dreamcast at RubyConf AU 2020!
See more here: https://youtu.be/ni-1x5Esa_o

Include time.h for ios

C++ is stricter in implicit type casting.

Differences from the PR #4940:
* Use simple search for short strings only.
* "short" means `m+n` is shorter than `MRB_QS_SHORT_STRING_LENGTH`.
* The current default value for `MRB_QS_SHORT_STRING_LENGTH` is 2048.

It is available by default in environments where `__unix__` is defined.
Other environments are enabled by defining `MRB_WITH_IO_PREAD_PWRITE`
(requires an implementation of `pread()` and `pwrite()` functions).

In any case, you can disable it by defining
`MRB_WITHOUT_IO_PREAD_PWRITE`.

Fix builds for old mingw in mruby-socket; ref #4914

that includes `float.h`. It allows definitions from native headers.

My cat stepped on the keyboard at the last moment before the commit.

Support bit flags for `IO.open`

In top-level, `mid` is `NULL`. We used to ignore 'mid` update for `NULL`.