```
coroutine/arm64/Context.S:31:57: error: invoking macro TOKEN_PASTE argument 1: empty macro arguments are undefined in ISO C90 [-Wpedantic]
   31 | .global PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer)
      |                                                         ^
```

- ISO C forbids conversion of function pointer to object pointer type
- ISO C forbids conversion of object pointer to function pointer type

Not enabling for ELF in general as not all platform support it
 (e.g. NetBSD, implictly stack never executable).

Saves comitters' daily life by avoid #include-ing everything from
internal.h to make each file do so instead.  This would significantly
speed up incremental builds.

We take the following inclusion order in this changeset:

1.  "ruby/config.h", where _GNU_SOURCE is defined (must be the very
    first thing among everything).
2.  RUBY_EXTCONF_H if any.
3.  Standard C headers, sorted alphabetically.
4.  Other system headers, maybe guarded by #ifdef
5.  Everything else, sorted alphabetically.

Exceptions are those win32-related headers, which tend not be self-
containing (headers have inclusion order dependencies).

We use COROUTINE_LIMITED_ADDRESS_SPACE to select platforms where address
space is 32-bits or less. Fiber pool implementation enables more book
keeping, and reduces upper limits, in order to minimise address space
utilisation.