Revert part of "Refine `tasks/toolchains/(gcc|clang).rake`" (07c6b7f0)

`-Wzero-length-array` is not a mandatory option, so the original is better.

Fix warnings for invalid C++ option with `enable_cxx_abi`; ref #3618, #4703

To stop warnings since C++ do not accept `-std=gnu99` option.

Refine `tasks/toolchains/(gcc|clang).rake`

- Make sure to specify `-std=gnu99` for C compiler flag.
- Make sure to specify `-Wzero-length-array` for C/C++ compiler flag (Clang).
- Extract similar codes.

1. `$/` and other Perl-ish global variables are not defined in ISO.
2. The current Ruby policy do not encourage those variables.
3. Those variables has global effect and can cause troubles.

Add paragraph mode to String#each_line in mrblib

The `Rakefile` was a leftover from the time it was an independent gem.

E.g. `io.read(5)` should read 5 byte string, not 5 characters.

Unlike CRuby, there's no way to process strings byte-wise by core
methods because there's no per string encoding in mruby, so that
we moved 3 byte-wise operation methods from `mruby-string-ext` gem.

Merge pull request #4701 from shuujii/drop-test-dependency-from-mruby-string-ext-to-mruby-enumerator

Drop test dependency from `mruby-string-ext` to `mruby-enumerator`

Refine documentation for C99 compliance; ref #4685 [ci skip]

The refinement was based on comment from @matz.

add special local variable _ in mirb

The fix was based on PR from @dearblue

Fix `Class.new` argument specs

Fix `mrb_vformat()` crashes with `MRB_INT16`

Optimize `mrb_bool()` with `MRB_WORD_BOXING`

If `MRB_INT16` is specified, the variable length argument `mrb_int` is
converted to `int`.

Since in mruby, Integer and Float interchange frequently (mostly on
overflow), so adding explicit `.0` can cause problems sometimes.

For example:
https://github.com/mattn/mruby-json/pull/40
https://github.com/pepabo/mruby-msd/pull/13
https://github.com/mattn/mruby-json/pull/42

Increase symbol bit size on 32-bit mode with `MRB_WORD_BOXING`

This change increases the max length of lower inline symbols to 5 bytes.

Avoid bit fields in `mrb_value`; ref b2c3d88f

The changes at b2c3d88f were inappropriate because the memory layout of bit
fields are implementation defined. Therefor, I fixed it not to use bit
fields.

Merge pull request #4691 from shuujii/fix-Range-max-test-TypeError-is-raised-on-32-bit-mode-with-word-boxing

Fix `Range#max` test (`TypeError` is raised) on 32-bit mode with word boxing