Fix #5177 by setting MRUBY_CONFIG early

Avoid warnings from clang's "-Wembedded-directive"

If it gives clang-11.0 `-Wembedded-directive`, a warning will be reported in `include/mruby/boxing_nan.h`.

```
include/mruby/boxing_nan.h:48:2: warning: embedding a directive within macro arguments has undefined behavior [-Wembedded-directive]
include/mruby/boxing_nan.h:52:2: warning: embedding a directive within macro arguments has undefined behavior [-Wembedded-directive]
```

The cause of this is #5117.
ref. e993b83c

Fix unintended variable shadowing

Raise an exception instead of exiting when `mrbc` fails

Change output condition in `_pp`

Merge pull request #5182 from shuujii/use-local-variables-instead-of-top-level-constants-in-boxing.rb

Use local variables instead of top level constants in `boxing.rb` [ci skip]

* Output even with `rake -v` (to help debugging)
* Not output when `rake -s`

Refine build log for generated files

Add brief explanation about `bin/mruby`; ref #5157 [ci skip]

* Output `GEN` log for generated files
* `MRBC` log is outputted one for each `mrbc` execution

#### Before this patch:

```console
CC    src/array.c -> build/host/src/array.o
(snip)
GEN   mrblib/*.rb -> build/host/mrblib/mrblib.c
      MRBC mrblib/00class.rb
      MRBC mrblib/10error.rb
(snip)
CC    mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
      MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC    mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
      MRBC mrbgems/mruby-socket/test/addrinfo.rb
      MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```

#### After this patch:

```console
GEN   build/presym
GEN   build/presym.inc
CC    src/array.c -> build/host/src/array.o
(snip)
GEN   mrblib/*.rb -> build/host/mrblib/mrblib.c
      MRBC mrblib/00class.rb
           mrblib/10error.rb
(snip)
CC    mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
GEN   build/host/mrbgems/mruby-time/gem_init.c
      MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC    mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
GEN   build/host/mrbgems/mruby-socket/gem_test.c
      MRBC mrbgems/mruby-socket/test/addrinfo.rb
      MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```

Move global function for build to `lib/mruby/core_ext.rb`

Fix GC mark leaks for `MRB_TT_BREAK`; fix #5168

Because `_pp` is originally defined in `lib/mruby/core_ext.rb`, other global
functions are moved to the file.

Fix size of local variable array in struct dumped file

Improved `Object#define_singleton_method`

Rename `MRB_{ENABLE,DISABLE}_` to `MRB_{USE,NO}_`; close #5163

Integrate the implementation with `Module#define_method`.

- Introduce the internal function `mrb_mod_define_method_m()` (no static)
- The `Object#define_singleton_method` method can now accept a second argument

|        Previous Name         |        New Name         |
|------------------------------|-------------------------|
| MRB_ENABLE_ALL_SYMBOLS       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_SYMBOLL_ALL       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_CXX_ABI           | MRB_USE_CXX_ABI         |
| MRB_ENABLE_CXX_EXCEPTION     | MRB_USE_CXX_EXCEPTION   |
| MRB_ENABLE_DEBUG_HOOK        | MRB_USE_DEBUG_HOOK      |
| MRB_DISABLE_DIRECT_THREADING | MRB_NO_DIRECT_THREADING |
| MRB_DISABLE_STDIO            | MRB_NO_STDIO            |
| ENABLE_LINENOISE             | MRB_USE_LINENOISE       |
| ENABLE_READLINE              | MRB_USE_READLINE        |
| DISABLE_MIRB_UNDERSCORE      | MRB_NO_MIRB_UNDERSCORE  |
| DISABLE_GEMS                 | MRB_NO_GEMS             |

* `MRB_ENABLE_SYMBOLL_ALL` seems to be a typo, so it is fixed.
* `MRB_` prefix is added to those without.
* The previous names can also be used for compatibility.

Allow to mixed and specify `*.rb` and `*.mrb` in `bin/mruby`

It is not decides by the extension.
In order to be recognized as a `.mrb` file, the following three points must be satisfied:
- File starts with "RITE"
- At least `sizeof(struct rite_binary_header)` bytes can be read
- `NUL` is included in the first 64 bytes of the file
If these are not met, it is judged as a text file and it is processed as a Ruby script.

The `bin/mruby -b` switch is still available which treats the given file as a `.mrb` file.

New `MRB_API` function:
- `include/mruby/compile.h` and `mrbgems/mruby-compiler/core/parse.y`
  - `mrb_load_detect_file_cxt()` (remove with `MRB_DISABLE_STDIO`)

NOTE:
- Even script files now always open in binary mode for `bin/mruby`.
  The `\r\n` is handled by the `nextc()` function already, so there is no problem even on Windows.
- The `nextc0()` function in `mrbgems/mruby-compiler/core/parse.y` can now specify a string buffer and a file pointer at the same time.
  In this case, get it from the string buffer first.

This patch includes modifies by comment of https://github.com/mruby/mruby/pull/5157.

Fix incorrect build name and simplify `build_config/boxing.rb` [ci skip]

`boxing-nan-m64` was duplicated.

Merge pull request #5164 from shuujii/automatically-enable-MRB_ENABLE_DEBUG_HOOK-if-mruby-bin-debugger-is-used

Automatically enable `MRB_ENABLE_DEBUG_HOOK` if `mruby-bin-debugger` is used

* Allow mixed declarations, especially loop variables
* Allow C++ style commends `//`

Fix documents for `mrb_ary_splice()` [ci skip]

Move some `.rake` files to `tasks` directory for consistency