Prohibit string changes by "s"/"z" specifier of `mrb_get_args()`

- The `s` specifier is a string pointer obtained without performing `mrb_str_modify()`, so it cannot be changed.
- The `z` specifier cannot be changed because it is a string pointer obtained by `RSTRING_CSTR()` which returns `const char *`.

examples/mrbgems: clarify the caller

Before this commit:

  mruby -e 'CRubyExtension.ruby_method' # => A Ruby Extension
  mruby -e 'CRubyExtension.c_method'    # => A C Extension
  mruby -e 'CExtension.c_method'        # => A C Extension
  mruby -e 'RubyExtension.ruby_method'  # => A Ruby Extension

After this commit:

  mruby -e 'CRubyExtension.ruby_method' # => CRubyExtension: A Ruby Extension
  mruby -e 'CRubyExtension.c_method'    # => CRubyExtension: A C Extension
  mruby -e 'CExtension.c_method'        # => CExtension: A C Extension
  mruby -e 'RubyExtension.ruby_method'  # => RubyExtension: A Ruby Extension

Remove redundant type-check

since mrb_str_to_str() also does it.

*.d format is version-independent

Fix skipping reading file

As pointed out by @shuujii on https://github.com/mruby/mruby/pull/5079 ,
multi-path in single-line could be appear even with older-gcc or clang.

Fix *.d parsing with gcc 9.3.0

Before this commit:

  $ rake
  ...
  $ rm build/host/src/gc.o
  $ rake
  rake aborted!
  Don't know how to build task '/home/foo/mruby/build/host/src/gc.o' (See the list of available tasks with `rake --tasks`)
  ...

After this commit:

  $ rake
  ...
  $ rm build/host/src/gc.o
  $ rake
  CC    src/gc.c -> build/host/src/gc.o
  ...

With gcc 9.3.0 on Ubuntu 20.04, build/host/src/array.d looks like:

  /build/host/src/array.o: /src/array.c \
   /include/mruby.h /include/mrbconf.h \
   /include/mruby/common.h \
   ...
   /include/mruby/range.h \
   /src/value_array.h

and it has been parsed to:

  [
                                           # /src/array.c missing
    "/include/mruby.h /include/mrbconf.h", # incorrectly parsed
    "/include/mruby/common.h",
    ...
    "/src/value_array.h",
  ]

After this change, *.d will be parsed correctly.

  [
    "/src/array.c",
    "/include/mruby.h",
    "/include/mrbconf.h",
    "/include/mruby/common.h",
    ...
    "/src/value_array.h",
  ]

Fix take over file scope variables with `mruby` and `mirb` command

Remove `enum call_type`

It seems to be unnecessary from mruby-1.0.0 or earlier.

mruby-io: Fixing compilation issue under the legacy MinGW environment

Reported by @shuujii.

Should have handled the case `to_a` returns `nil`.

Adding MRB_MINGW32_LEGACY in common.h in order to identify the legacy MinGW environment (i.e. NOT to be confused with MinGW-w64).
For more info about MinGW defined macros, see: https://sourceforge.net/p/predef/wiki/Compilers/

Simplify MSVC detection to `mrb_static_assert`

Merge pull request #5062 from shuujii/use-normal-static_assert-in-mrb_static_assert-as-much-as-possible

Use normal `static_assert` in `mrb_static_assert` as much as possible

Use `struct _stat32` instead of `struct __stat32`

It is described as `struct __stat32` in the MSVC reference manual.
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/stat-functions?view=vs-2019
But it doesn't really exist, so it must use `struct _stat32`.

It also replaces `struct __stat64` with `struct _stat64` to make it look
nicer.

* `_Static_assert` can also be used with `-std=gnu99` on GCC >= 4.6.
* `static_assert` can be used on MSVC.
* `static_assert` can be used even on old G++/Clang++ if
  `__GXX_EXPERIMENTAL_CXX0X__` is defined.

Allow `mrb_static_assert()` to be used outside of functions

The use of `struct` is an idea by @dearblue.