When setjmp/longjmp/exceptions are used on Windows it's necessary to store+restore additional information from the TEB.
I didn't find any official documentation about the values to be saved, but found the corresponding boost/context implemenataion:
    https://github.com/boostorg/context/commit/abf8e04e23cf05a499594e674d1c90db39117662

This is similar to the special TIB handling on x86/x86_64 on Windows.

Without this fix an exception in a fiber segfaults without any output:
  ruby -e "Fiber.new{ raise 'test' }.resume"

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

With `--std=c99` option coroutine/arm64/Context.h errs:

```
In file included from cont.c:26:
coroutine/arm64/Context.h:59:5: error: call to undeclared function 'asm'; ISO C99 and later do not support
      implicit function declarations [-Wimplicit-function-declaration]
   59 |     asm ("hint #8;" : "+r"(r17) : "r"(r16));
      |     ^
```

Also move the common function header.

We don't need to save/restore x30 twice, and we can just use `ret`,
which uses x30 as return address register instead of explicit `ret <reg>`
instruction. This also allows us to use `autiasp` instead of `autia1716`
and we can skip setting SP/LR to x16/x17.

Also the size of register save area is shrunk by 16 bytes due to the
removal of extra x30 save/restore.