Fixed build NaN boxing with 32-bit CPU mode

`SET_CPTR_VALUE()` requires the `p` field on 32-bit CPU mode.

Suppress `-Wunused-function`

If no gem was specified, it was warning because `init_mrbgems()` was not used.

ref: #4250

That loads 32 bit integer bypassing pool access.

Reduce recompilation when updating presym target files

Fix dependencies of `y.tab.o`

Fix that updating presym target files would recompile almost all files even
if symbols did not change.

In the case of `y.tab.o`, `y.tab.d` was not read, so `y.tab.c` was not
recompiled by updating header files.

It seemed that the cause was overwriting the rake rule, so I deleted
unnecessary settings.

New instructions:
  * OP_LOADL16
  * OP_LOADSYM16
  * OP_STRING16

Size of pools, symbols are `int16_t` but offset representation in the
bytecode was 8 bits. Size of child `irep` array is `int16_t`, too but
this change does not address it.

`minirake` was still used in many occasions.
This reverts commit 3847c7eb.

Since `R-assignment` in CRuby is abandoned.  Single-line pattern matching
in `mruby` only matches single local variable at the moment. Currently
it works as a right assignment to a local variable. It will be enhanced
in the future.

Fix parallel build

When `rake -m` and so on are used to build in parallel, building may be
started before presym files are generated. Then, for example, the following
error occurs and this issue is fixed.

```console
In file included from /Users/shuujii/mruby/mruby/include/mruby.h:92:
/mruby/mruby/include/mruby/presym.h:16:10: fatal error: '../build/presym.inc' file not found
#include <../build/presym.inc>
         ^~~~~~~~~~~~~~~~~~~~~
```

ossfuzz: Fix mruby include paths

Add `preview` to version.

If you confirm the gem is cdump safe, add `spec.enable_cdump` in
`mrbgem.rake` file. Some external gems e.g. `mruby-mgem-template` do
not work with cdump.