It has been used only for DOSISH other than Windows.

`rb_jmpbuf_t` type is considerably large due to inline-allocated
Asyncify buffer, and it leads to stack overflow even with small number
of C-method call frames. This commit allocates the Asyncify buffer used
by `rb_wasm_setjmp` in heap to mitigate the issue.

This patch introduces a new type `rb_vm_tag_jmpbuf_t` to abstract the
representation of a jump buffer, and init/deinit hook points to manage
lifetime of the buffer. These changes are effectively NFC for non-wasm
platforms.

Just as the state was originally passed as the return value of
`longjmp`, the state stored in EC cannot be 0 when the jump is
captured now too.

[Misc #18891]

In a forked process from a fiber, the fiber becomes the only
fiber, `fiber_switch` does nothing as there is no other fibers,
`rb_fiber_terminate` does not terminate the fiber.  In that case,
reaches the end of `fiber_entry` finaly, which is declared as
"COROUTINE" and should never return.