* In explanation of mruby, the expression `build_config.rb` is frequently
  used including official documents, so I think that it will not make sense
  if the file is no longer used.
* The `MRUBY_TARGET` mechanism seems to have little improvement, so I don't
  think it should be changed to avoid unnecessary confusion.
* `MRUBY_TARGET` and `MRuby.targets` represent somewhat different things,
  so using the same term "target" is a bit confusing.

The mechanism that can be written short when using a file under
`build_config` (renamed from `target`) directory remains
(`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
in `MRUBY_CONFIG` doesn't exist).

Ref #5093; close #5085

Fix for #5093

It was once turned off by #4045

Thanks to @YunzheZJU

According to `doc/guides/compile.md`; thanks to @YunzheZJU

mruby 3.0.0-preview.

Mruby3

Files under `test/t` and `mrbgem/*/test` are for tests, not for actual
execution. So symbols in those files need not to be pre-allocated. This
change slightly reduce the memory consumption.

Co-Authored-By: n4o847 <22975590+n4o847@users.noreply.github.com>
Co-Authored-By: smallkirby <ssmallkirby@gmail.com>

Remove unnecessary assignment in String#upto

This reverts commit 8746a6fe4e7bda8a0fbc0eaece9314ec51a0c255.

We already have `mrb_protect()`, `mrb_ensure()` and `mrb_rescue()`
functions. If you need to handle exceptions from C functions, use those
functions above.

`mrb_exc_protect()` takes two C functions, `body` to be executed first,
and `resc` to be executed when an error happens during `body` execution.
Since `mrb_exc_protect()` should be compiled with the proper compiler,
we will not see the problem like #5088 that was caused by `setjmp()` and
`throw` mixture.

`MRB_TRY()` does not work when compiled by C compiler with `cxx_exception`
configuration. We should explicitly warn.

`MRB_TRY()` does not work when compiled by C compiler with `cxx_exception`,
due to the mixture of `setjmp()` used by `mirb.c` and `throw` used by
the core.  The original intension of e2e6554b is to protect code from
signal interruption, but the signal interruption is not well-defined in
mruby anyway.

Also fixed the size calculation of `irep` dump, that could cause memory
corruption.

To allow C++ compilation. Fix suggested by @dearblue.

Redirect `mrb_str_to_str` to `mrb_obj_as_string` via C macro.
Inspired by #5082

Those types are `uint16_t` in definition. Also we no longer need padding
for `iseq`.

The fix was proposed by @dearblue

The fix was proposed by @dearblue

The fix was proposed by @dearblue

Need to support `int / int -> int` update.

Target `host-debug` to use `host` internal target with debugging
configuration.

The PR was from @cubicdaiya.